Coordination properties of 2,5-dimesitylpyridine: An encumbering and versatile ligand for transition-metal chemistry

Article abstract of DOI:10.1021/ic201205p

To overcome the unfavorable steric pressures associated with 2,6-disubstitution in encumbering pyridine ligands, the coordination chemistry of a 2,5-disubstituted variant, namely, 2,5-dimesitylpyridine (2,5-Mes ₂py), is reported. This diaryl pyridine shows good binding ability to a range of transition-metal fragments with varying formal oxidation states and coligands. Treatment of 2.0 equiv of 2,5-Mes₂py with monovalent Cu and Ag triflate sources generates complexes of the type [M(2,5-Mes ₂py)₂]OTf (M = Cu, Ag; OTf = OSO₂CF ₃), which feature long M-OTf distances and a substrate-accessible primary coordination sphere. Combination of 2,5-Mes₂py with Cu(OTf)₂ and Pd(OAc)₂ produces four-coordinate complexes featuring cis- and trans-2,5-Mes₂py orientations, respectively. The four-coordinate palladium complex Pd(OAc)₂(2,5-Mes ₂py)₂ is found to resist py-ligand dissociation at room temperature in solution, but functions as a precatalyst for the aerobic C-H bond olefination of benzene at elevated temperatures. This C-H bond activation chemistry is compared with a similar Pd-based system featuring 2,6-disubstituted pyridines. 2,5-Mes₂py also readily supports mono- and dinuclear divalent Co complexes, and the solution-phase equilibria between such species are detailed. The coordination studies presented highlight the potential of 2,5-Mes₂py to function as an encumbering ancillary for the stabilization of low-coordinate complexes and as a supporting ligand for metal-mediated transformations.

Full text of DOI:10.1021/ic201205p

ARTICLE
pubs.acs.org/IC
Coordination Properties of 2,5-Dimesitylpyridine: An Encumbering
and Versatile Ligand for Transition-Metal Chemistry
Julia M. Stauber, Andrew L. Wadler, Curtis E. Moore, Arnold L. Rheingold, and Joshua S. Figueroa*
Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla,
California 92093-0358, United States
S Supporting Information
b
ABSTRACT: To overcome the unfavorable steric pressures
associated with 2,6-disubstitution in encumbering pyridine
ligands, the coordination chemistry of a 2,5-disubstituted
variant, namely, 2,5-dimesitylpyridine (2,5-Mes2py), is re-
ported. This diaryl pyridine shows good binding ability to a
range of transition-metal fragments with varying formal oxida-
tion states and coligands. Treatment of 2.0 equiv of 2,5-Mes2py
with monovalent Cu and Ag triflate sources generates complexes of the type [M(2,5-Mes2py)2]OTf (M = Cu, Ag; OTf =
OSO2CF3), which feature long M-OTf distances and a substrate-accessible primary coordination sphere. Combination of 2,5-
Mes2py with Cu(OTf)2 and Pd(OAc)2 produces four-coordinate complexes featuring cis- and trans-2,5-Mes2py orientations,
respectively. The four-coordinate palladium complex Pd(OAc)2(2,5-Mes2py)2 is found to resist py-ligand dissociation at room
temperature in solution, but functions as a precatalyst for the aerobic CꢀH bond olefination of benzene at elevated temperatures.
This CꢀH bond activation chemistry is compared with a similar Pd-based system featuring 2,6-disubstituted pyridines. 2,5-Mes2py
also readily supports mono- and dinuclear divalent Co complexes, and the solution-phase equilibria between such species are
detailed. The coordination studies presented highlight the potential of 2,5-Mes2py to function as an encumbering ancillary for the
stabilization of low-coordinate complexes and as a supporting ligand for metal-mediated transformations.
’ INTRODUCTION
6-Dipp2C6H3 m-terphenyl ligand (Dipp = 2,6-(i-Pr)2C6H3)21,22
enabled Power’s discovery of the CrꢀCr dimer, Cr2(2,6-Dipp2-
C6H3)2, which was the first example of a complex possessing
5-fold bonding between two metal centers.14,23ꢀ27
As a result of its good binding ability and the moderately
strong ligand field it provides, pyridine (py) is a ubiquitous ligand
in transition-metal chemistry. While many substituted, mono-
dentate pyridines have found wide utility as ligands, sterically
encumbering 2,6-diaryl pyridines have received little attention.
Indeed, for this class of pyridine ligands, only two variants have
been reported, namely, the 2,6-dimesityl and 2,6-triisopropyl-
phenyl derivatives, 2,6-Mes2py and 2,6-Tripp2py, respectively
(Mes = 2,4,6-Me3C6H2; Tripp = 2,4,6-(i-Pr)3C6H2).1,2 However,
the transition-metal coordination chemistry of these ligands
is limited to the monovalent-silver complex, [Ag(2,6-Me-
s2py)2]OTf (OTf = trifluoromethanesulfonate).1 This lack of
attention is not surprising given that 2,6-lutidine (2,6-Me2py),
although able to form isolable complexes,3ꢀ8 binds transition
metals with markedly low affinity because of the steric pressures
posed by its 2,6-methyl groups.9,10
Although underexplored, the 2,6-diaryl pyridine framework
is attractive because of its topological relationship to sub-
stituted m-terphenyl σ-aryl ligands ([2,6-Ar2C6H3]ꢀ; Ar = aryl,
Chart 1),11ꢀ14 which have been successfully employed as en-
cumbering ancillaries for low-coordinate transition-metal
complexes.15ꢀ17 Indeed, use of the common, 2,6-dimesityl-
substitued m-terphenyl ligand (i.e., 2,6-Mes2C6H3)18 has allowed
for the isolation and structural characterization of two-coordinate,
divalent Mn, Fe, and Co complexes of the simple formula-
tion M(Ar)2.19,20 Further, use of the more encumbering 2,
Encumbering diaryl pyridines may similarly be expected to
foster low-coordination numbers, but in a manner that does not
require a metal valence unit for bonding. This feature is important
for preserving additional redox equivalents for multielectron,
metal-based transformations or catalysis. Thus, in an effort to
avoid the potential binding limitations of 2,6-aryl pyridines, we
reasoned that 2,5-diaryl substitution of a pyridine ring might
maintain the encumbering environment offered by the terphenyl
framework, while minimizing the steric pressures that lead to
poor pyridine binding (Chart 1). Accordingly, herein we report the
efficacy of this substitution strategy by introducing the 2,5-dimesityl
pyridine derivative, 2,5-Mes2py (Mes = 2,4,6-Me3C6H2), and
show that it can support a range of structurally diverse transition-
metal complexes and CꢀH bond functionalization catalysis.
’ RESULTS AND DISCUSSION
Scheme 1 shows a straightforward route to 2,5-dimesitylpyr-
idine based on Kumada-type cross-coupling. This procedure is
similar to that reported for 2,6-dimesitylpyridine and provides
the desired 2,5-Mes2py ligand in 71% yield when employing
Received: April 28, 2011
Published: June 28, 2011
r
2011 American Chemical Society
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Chart 1. Topological Correlation between m-Terphenyl, 2,6-
Scheme 2
Diaryl Pyridine, and 2,5-Diaryl Pyridine Ligandsa
aThe dimesityl derivative of each is used as an example.
Scheme 1
about 5.0 g of 2,5-dibromopyridine. Upon isolation, Mes2py is
obtained as a colorless, air-stable crystalline solid that is freely
soluble in common organic solvents. Crystallographic character-
ization of 2,5-Mes2py was complicated by end-over-end posi-
tional disorder of the nitrogen atom and para CꢀH unit and thus
inhibited an accurate determination of the metrical parameters
for the py ring (Supporting Information, Figure S3). However,
the symmetrically distinct mesityl groups in 2,5-Mes2py are
readily apparent in its 1H NMR spectrum (C6D6), which displays
nine unique resonances. The solution IR spectrum of 2,5-Mes2py
(C6D6) exhibits moderately strong bands at 1610, 1587, and
1467 cmꢀ1 which are in the range and intensity expected for
CdN and CdC stretching modes.28
In analogy to its 2,6-substituted counterpart, 2,5-Mes2py
readily forms a bis-pyridine complex with monovalent Ag
centers. As shown in Scheme 2, treatment of 2,5-Mes2py with
AgOTf in tetrahydrofuran (THF) solution provides the salt
[Ag(2,5-Mes2py)2]OTf (1) as determined by X-ray diffraction
(Figure 1). Interestingly, the solid-state structure of 1 possesses
markedly long bond distances between the Ag center and the
OTfꢀ oxygen atoms (Ag1ꢀO1 = 2.876(2) Å, Ag1ꢀO2 =
2.779(2) Å). While we contend that these AgꢀO contacts are
non-negligible, they are near the limit of a bonding interaction
between the metal center and the OTf anion and likely reflect the
strong σ-donating properties of the 2,5-Mes2py ligand.29 How-
ever, whereas the metal primary coordination sphere is inacces-
sible to the OTfꢀ ion in [Ag(2,6-Mes2py)2]OTf,1 two 2,5-
Mes2py ligands clearly allow close approach of the counterion
to the Ag center. This feature is significant in that both [Ag(2,6-
Mes2py)2]OTf and 1 maintain near-linear NꢀAgꢀN angles
(178.1(2)° vs 164.29(7)°, respectively), but differ in their ability
to accommodate additional ligands. We believe this highlights
the prospects for substrate activation chemistry in the protected,
yet accessible, 2,5-Mes2py system.
Figure 1. Molecular Structure of [Ag(2,5-Mes2py)2]OTf (1). Selected
distances (Å) and angles (deg): Ag1ꢀN1 = 2.1556(19), Ag1ꢀN2 =
2.1523(19), Ag1ꢀO1 = 2.876(2), Ag1ꢀO2 = 2.779(2), N1ꢀAg1ꢀN2 =
164.29(7).
THF solution generates the colorless complex [Cu(2,5- Mes2py)2]-
OTf (2) as determined by X-ray diffraction (Scheme 2, Figure 2).
The overall structural features of 2 are analogous to its Ag con-
gener 1, and it similarly possesses a long interaction between the
monovalent metal center of the OTfꢀ counterion (d(Cu1ꢀO1) =
2.423(3) Å). As expected, Cu-OTf distances contract substan-
tially when the valence of the metal center is increased. Thus
treatment of 2 equiv of 2,5-Mes2py with the divalent copper
source Cu(OTf)2 in CH2Cl2 solution exclusively provides the cis-
isomer of Cu(OTf)2(2,5-Mes2py)2 (cis-3) as a green paramagnetic
crystalline solid (μeff = 1.8(1) μB, Evans Method, CDCl3/
(Me3Si)2O, 20 °C, Scheme 2). Structural characterization of
cis-3 revealed Cu1ꢀO1 and Cu1ꢀO2 distances of 1.989(3) Å
and 1.945(3) Å, respectively, which are indicative of CuꢀO single
bonds (Figure 3). Notably, a slight elongation of the CuꢀN
distances is observed in cis-3 relative to 2 (1.990 Å (av) vs 1.907 Å
Most importantly, 2,5-Mes2py can bind to a range of other
transition metals and is compatible with several other ligands. For
example, treatment of 2,5-Mes2py with (μ-C6H6)[Cu(OTf)]2 in
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Figure 4. Molecular Structure of Pd(OAc)2(2,5-Mes2py)2 (trans-4).
Selected distances (Å) and angles (deg): Pd1ꢀO1 = 2.0151(17),
Pd1ꢀO2 = 2.0078(18), Pd1ꢀN1 = 2.029(3), Pd1ꢀN2 = 2.031(3),
O1ꢀPd1ꢀO2 = 176.00(7), N1ꢀPd1ꢀN2 = 171.91(11), O1ꢀPd1ꢀ
N1 = 87.84(12), O1ꢀPd1ꢀN2 = 91.94(12), O2ꢀPd(1)-N1 = 93.17(11),
O2ꢀPd1ꢀN2 = 87.60(11).
Figure 2. Molecular Structure of [Cu(2,5-Mes2py)2]OTf (2). Selected
distances (Å) and angles (deg): Cu1ꢀN1 = 1.900(3), Cu1ꢀN2 =
1.906(3), Cu1ꢀO2 = 2.423(3), N1ꢀCu1ꢀN2 = 164.05(11), N1ꢀ
Cu1ꢀO2 = 100.71(10), N2ꢀCu1ꢀO2 = 94.99(10).
acetate. Addition of 2 equiv of 2,5-Mes2py to Pd(OAc)2 in
CH2Cl2 solution provides a mixture of two species in a 20:1 ratio.
Selective crystallization from Et2O solution, followed by single-
crystal X-ray diffraction revealed the major product to be
trans-Pd(OAc)2(2,5-Mes2py)2 (trans-4, Scheme 2, Figure 4).
Although not isolated from the reaction mixture, we presume
that the minor product is the cis-isomer of Pd(OAc)2(2,5-
Mes2py)2, because it exhibits a 1:1 ratio of acetate to 2,5-Mes2py
1H NMR resonances.32 When monitored over the course of 5
days, these complexes do not interconvert in solution (C6D6) in
a manner that alters the initial product distribution.
Monomeric trans-4 is most structurally comparable to the
complex Pd(OAc)2(2,6-i-Oct2py)2 (i-Oct = isooctyl) reported
by Yu in the context of Pd-catalyzed aerobic arylꢀC-H bond
olefination.33 Interestingly, 2,6-dialkyl substituted Pd(OAc)2-
(2,6-i-Oct2py)2 was proposed to undergo a ligand-dissocia-
tion/dimerization sequence in CDCl3 solution at room
temperature to the bridging-acetate dimer [μ-(k2-OAc)Pd-
(OAc)(2,6-i-Oct2py)]2. The latter results from dimerization of
the three-coordinate, monopyridine intermediate [Pd(OAc)2-
(2,6-i-Oct2py)], which is presumably responsible for the electro-
philic CꢀH activation of aryl substrates during catalysis.34ꢀ36 In
contrast, trans-4 is stable for extended periods at room tempera-
ture in both CDCl3 and C6D6 solution, which further high-
lights the difference in steric pressure between 2,6- and 2,
5-substitution of the pyridine framework. Interestingly, heating
of trans-4 in C6D6 solution to 60 °C for 2 h results in about 15%
decomposition to free 2,5-Mes2py (1H NMR), with insoluble
black precipitates noticeable in the reaction mixture. However,
thermolysis does not result in the formation of an observable new
species as assayed by 1H NMR spectroscopy, and full decomposi-
tion is achieved under these conditions after approximately 24 h.
Despite the fact that ligand dissociation is not readily observed
from trans-4, it can indeed catalyze aerobic CꢀH bond olefina-
tion. Accordingly, benzene and ethyl acrylate are readily coupled
to trans-ethyl cinnamate in 74% isolated yield using 10 mol % of
trans-4 in the presence of acetic anhydride (Ac2O) and O2
(Scheme 3).37,38 When using benzene as both the substrate
and the solvent, this transformation occurs at 60 °C over the
course of 6 h, which represents fairly mild conditions for an
Figure 3. Molecular Structure of Cu(OTf)2(2,5-Mes2py)2 (cis-3).
Selected distances (Å) and angles (deg): Cu1ꢀO1 = 1.989(3), Cu1ꢀ
O2 = 1.945(3), Cu1ꢀN1 = 1.995(4), Cu1ꢀN2 = 1.985(4), O1ꢀCu1ꢀ
O2 = 87.54(15), N1ꢀCu1ꢀN2 = 99.47(16), O1ꢀCu1ꢀN1 =
164.48(16), N1ꢀCu1ꢀO2 = 84.39(15), O2ꢀCu1ꢀN1 = 91.99(16),
O2ꢀCu1ꢀN2 = 163.93(16).
(av)). While this lengthening may reflect electronic destabiliza-
tion between the nitrogen atoms and the Cu(II) singly occupied
dx2ꢀy2 orbital, steric pressures between the cis-disposed 2,5-
Mes2py ligands likely also contribute to the observed CuꢀN
bond distances in cis-3. Supporting this notion is the fact that the
related Cu(II) bis-triflate, bis-oxazoline complex Cu(OTf)2(k2-
N,N-(S,S-i-Pr2boxBz2))30 (S,S-i-Pr2boxBz2 = [(4S,40S)-4,40,5,50-
tetrahydro-4,40-diisopropyl-2,20-(dibenzylmethylene)dioxazole])
exhibits CuꢀN bond distances (d(CuꢀN) = 1.941 Å (av)) that
are slightly shorter than those in cis-3. We believe these shorter
distances arise because the planar, cis-chelating S,S-i-Pr2boxBz2
ligand poses less steric encumbrance around the Cu center
relative to two cis-disposed 2,5-Mes2py units. Interestingly, cis-
3 and Cu(OTf)2(k2-N,N-(S,S-i-Pr2boxBz2)) represent the only
four-coordinate Cu(II) bis-triflate complexes supported by ni-
trogen-based ligands to be structurally characterized, whereas
many five- and six-coordinate complexes have been reported.31
In contrast to cis-3, a trans-disposed complex is dominant
upon coordination of two 2,5-Mes2py ligands to palladium(II)
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Scheme 3
Scheme 4
Figure 5. Molecular Structure of [(μ-I)CoI(2,5-Mes2py)]2 (5, note
crystallographic inversion symmetry). Selected distances (Å) and angles
(deg): Co1ꢀI1 = 2.5497(9), Co1ꢀI2 = 2.6284(9), Co1ꢀN1 =
2.033(4), Co1ꢀI20 = 2.6533(9), I2ꢀCo10 = 2.6533(9), Co1ꢀI2ꢀCo10 =
83.79(3), N1ꢀCo1ꢀI1 = 105.38(13), N1ꢀCo1ꢀI2 = 125.70(12),
I1ꢀCo1ꢀI2 = 110.63(3), N1ꢀCo1ꢀI20 = 100.40(11), I1ꢀCo1ꢀI20 =
118.96(3), I2ꢀCo1ꢀI20 = 96.21(3).
aerobic CꢀH bond olefination reaction.32,39ꢀ43 Thus, we believe
that the thermolysis study of trans-4 in C6D6 above results in the
formation of products stemming from C-D activation, which
decompose in the absence of arene-coupling partners and
additives. Notably, 1:2 in situ mixing of Pd(OAc)2 and 2,5-
Mes2py (10 mol % [Pd]), the approach typical for such olefina-
tion reactions, results in slightly lower yield for the coupling of
benzene and ethyl acrylate relative to preformed trans-4 under
similar conditions (67%).
The encumbering 2,5-Mes2py ligand also displays intriguing
coordination properties and solution behavior in conjunction
with divalent cobalt halides. For example, treatment of CoI2 with
1 equiv of 2,5-Mes2py in THF solution results in the green,
bridging-diiodide dimer, [(μ-I)CoI(2,5-Mes2py)]2 (5) as deter-
mined by X-ray diffraction (Scheme 4, Figure 5). Interestingly,
Figure 6. Molecular Structure of CoI2(2,5-Mes2py)2 (6, only one
crystallographically, but chemically equivalent, molecule shown). Se-
lected distances (Å) and angles (deg): Co1ꢀI1 = 2.5658(10), Co1ꢀI2 =
2.5495(10), Co1ꢀN1 = 2.074(5), Co1ꢀN2 = 2.082(5), N1ꢀCo1ꢀN2 =
92.3(2), N1ꢀCo1ꢀI2 = 103.11(14), N1ꢀCo1ꢀI1 = 125.56(15), N2ꢀ
Co1ꢀI1 = 100.27(15), N2ꢀCo1ꢀI2 = 126.44(16), I2ꢀCo1ꢀI1 =
110.38(4).
bridging diiodide 5 in C6D6 creates an equilibrium mixture
between the starting materials and the paramagnetic monomer,
CoI2(2,5-Mes2py)2 (6), with Keq = 1.0(1) at 25 °C.45 Addition of
4 equiv of 2,5-Mes2py to dimer 5 shifts the equilibrium constant
to Keq = 1.7(4), which is sufficient to isolate 6 by crystallization
from an Et2O/THF mixture at ꢀ35 °C. Crystallographic char-
acterization of 6 confirmed its identity (Figure 6) and revealed a
four-coordinate cobalt center that deviates significantly from
ideal tetrahedral symmetry according to Houser’s τ4 four-co-
ordinate geometry index (τ4 geometry index = 0.79 (av)).44,46
The speciation of cobalt-2,5-Mes2py complexes can also be
modulated by the Lewis acidity properties of the Co center.
Whereas equimolar mixtures of 2,5-Mes2py and CoI2 form dimer
5 in THF solution, a similar protocol employing CoBr2 produces
the monomer CoBr2(THF)(2,5-Mes2py) (7) as the exclusive
product (Scheme 4). We believe that the greater Lewis
acidity47,48 of the cobalt center in CoI2 is not effectively
quenched by THF relative to CoBr2, and thus results in dimer
formation. However, when 7 is dissolved in C6D6, free THF is
apparent in the 1H NMR spectrum and a μeff value of 6.3(2) μB is
Evans Method magnetic moment determination on
5
(C6D6/(Me3Si)2O, 20 °C) resulted in a μeff value of 5.9(2) μB,
which indicates an overall S = 2 ground state for the dimer.
Nominally tetrahedral divalent Co centers are well-known to
exhibit quartet, S = 3/2 ground states when ligated to weak to
moderate field ligands.44 Thus, the magnetic data for 5 suggest
that it features antiferromagnetic coupling of two unpaired spins
across the bridging-diiodide core and, correspondingly, that its
dimeric formulation remains intact in solution.
Notably, 5 is reminiscent of the bridging diacetate dimer
[μ-(k2-OAc)Pd(OAc)(2,6-i-Oct2py)]2 spectroscopically ob-
served by Yu upon ligand dissociation from trans-Pd(OAc)2-
(2,6-i-Oct2py)2. While similar behavior is not observed for the
palladium complex trans-4, dimer 5 does engage in ligand
association/dimerization equilibria in the presence of added
2,5-Mes2py. Accordingly, addition of 2 equiv of 2,5-Mes2py to
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Scheme 5
other 2,5-diaryl pyridines may be particularly useful in this regard
and are exploring further the reactivity of their transition-metal
complexes.
’ EXPERIMENTAL SECTION
General Considerations. All manipulations were carried out
under an atmosphere of dry dinitrogen using standard Schlenk and
glovebox techniques. Solvents were dried and deoxygenated according
to standard procedures. Unless otherwise stated, reagent grade starting
materials were purchased from commercial sources and used as received
or purified by standard procedures. Benzene-d6 and chloroform-d
(Cambridge Isotope Laboratories) were degassed and stored over 4 Å
molecular sieves for 2 d prior to use. Celite 405 (Fisher Scientific) was
dried under vacuum (24 h) at a temperature above 250 °C and stored in
the glovebox prior to use.
Solution 1H, 13C{1H}, and 19F{1H} spectra were recorded on Varian
Mercury 300 and 400 spectrometers, a Varian X-Sens500 spectrometer
or a JEOL ECA-500 spectrometer. 1H and 13C{1H} chemical shifts are
reported in ppm relative to SiMe4 (1H and 13C δ = 0.0 ppm) with
reference to residual solvent resonances of 7.16 ppm (1H) and 128.06
ppm (13C) for benzene-d6 and 7.26 ppm (1H) for chloroform-d.52
19F{1H}NMR spectra were referenced externally to neat trifluoroacetic
acid, F3CC(O)OH (δ = ꢀ78.5 ppm vs CFCl3 = 0.0 ppm). FTIR spectra
were recorded on a Thermo-Nicolet iS10 FTIR spectrometer. Samples
were prepared as C6D6 solutions injected into a ThermoFisher solution
cell equipped with KBr windows. For solution FTIR spectra, solvent
peaks were digitally subtracted from all spectra by comparison with an
authentic spectrum obtained immediately prior to that of the sample.
The following abbreviations were used for the intensities and character-
istics of important IR absorption bands: vs = very strong, s = strong,
m = medium, w = weak, vw = very weak; b = broad, vb = very broad,
sh = shoulder. Combustion analyses were performed by Robertson
Microlit Laboratories of Madison, NJ (U.S.A.).
Figure 7. Molecular Structure of CoBr2(THF)(2,5-Mes2py) (7). Se-
lected distances (Å) and angles (deg): Co1ꢀO1 = 1.989(3), Co1ꢀN1 =
2.040(3), Co1ꢀBr1 = 2.3959(9), Co1ꢀBr2 = 2.3638(9), O1Sꢀ
Co1ꢀN1 = 124.41(14), O1ꢀCo1ꢀBr2= 105.10(10), O1ꢀCo1ꢀBr1=
103.92(10), N1ꢀCo1ꢀBr1= 101.60(10), N1ꢀCo1ꢀBr2 = 101.30(10),
Br1ꢀCo1ꢀBr1 = 122.31(3).
Synthesis of 2,5-Dimesitylpyridine (2,5-Mes2py). To a THF
solution of 2,5-dibromopyridine (5.380 g, 0.0227 mol, 100 mL) and
PdCl2(PPh3)2 (0.798 g, 0.00114 mol, 5 mol %) was added a 1 M THF
solution of MesMgBr (0.050 mol, 50 mL) dropwise at 0 °C. Upon
complete addition, the reaction mixture was allowed to warm to room
temperature and then heated under reflux for 7 h at 75 °C. The reaction
mixture was quenched by the dropwise addition of H2O (20 mL) under
an N2 flow, and then poured over H2O (200 mL). The organic materials
were extracted with EtOAc (3 ꢁ 150 mL), washed with brine (20 mL),
and then dried over MgSO4. All volatile materials were then removed in
vacuo. Flash chromatography using a 93:7 hexanes/EtOAc mixture
provided a colorless residue. Crystallization of this material from
hexanes at ꢀ20 °C, followed by thorough drying in vacuo, provided
pure 2,5-dimesitylpyridine (2,5-Mes2py) as a colorless crystalline solid.
Yield: 5.085 g, 0.016 mol, 71.0%. 1H NMR (400.1 MHz, C6D6, 20 °C):
δ = 8.64 (dd, 1H, 4J = 2 Hz, 5J =1 Hz, o-py), 7.11 (dd, 1H, 3J = 8 Hz, 4J =
2 Hz, p-py), 6.94 (dd, 1H, 3J = 8 Hz, 5J = 1 Hz, m-py), 6.89 (s, 2H, m-Mes),
6.83 (s, 2H, m-Mes), 2.22 (s, 3H, p-CH3-Mes), 2.19 (s, 3H, p-CH3-
Mes), 2.18 (s, 6H, o-CH3-Mes), 1.99 (s, 6H, o-CH3-Mes) ppm. 13C{1H}
NMR (100.6 MHz, C6D6, 20 °C): δ = 159.0, 150.6, 138.8, 137.3, 137.2,
136.8, 136.2, 136.0, 135.8, 134.6, 128.7 (two resonances overlapping)
124.4, 21.2 (p-CH3-Mes), 21.2 (p-CH3-Mes), 20.9 (o-CH3-Mes), 20.5
(o-CH3-Mes) ppm. FTIR (C6D6, KBr windows): 3012 (m), 2968 (m),
2945 (m), 2919 (m), 2855 (m), 1610 (m), 1587 (m), 1543 (m), 1488
(m), 1467 (s), 1374 (w), 1365 (w, sh), 1328 (w), 1046 (vw), 1018 (w),
1000 (w), 849 (m) cmꢀ1. FTIR (KBr pellet): 3009 (m), 2967 (m), 2915
(m), 2855 (m), 2733 (w), 1612 (m), 1590 (m), 1546 (w), 1467 (s),
1375 (w), 1287 (w), 1123 (w), 1050 (w), 1021 (m), 998 (w), 945 (vw),
844 (m), 736 (w), 645 (w), 615 (w), 570 (m) cmꢀ1. Anal. Calcd. for
C23H25N: C, 87.56; H, 7.99; N, 4.44. Found: C, 87.29; H, 8.24; N, 4.44.
obtained. We believe these observations are indicative of the
formation of the S = 2 dimer, [(μ-Br)CoBr(2,5-Mes2py)]2 (8),
1
in absence of excess THF (Scheme 5). Indeed, the H NMR
spectrum of monomer 7 in C6D6 solution is nearly identical to
that of the diiodide dimer 5, which further suggests that
dimerization to 8 occurs when THF is not present in excess.49
Nevertheless, the monomeric nature of 7 as obtained from THF
solution is intriguing (Figure 7), as only one other example of a
neutral monomer with the relatively simple formulation CoX2-
(THF)L has been structurally characterized (X = halide).50,51
’ CONCLUDING REMARKS
In conclusion, 2,5-dimesityl substitution results in a diaryl
pyridine that is capable of ligating a range of transition-metal
fragments. The structural properties of the complexes reported
here demonstrate that the 2,5-Mes2py ligand provides an en-
cumbering steric environment, but also readily allows substrate
access to central metal atoms. In this respect, 2,5-diaryl substitu-
tion of a pyridine ring is an effective strategy for minimizing the
steric pressures that are known to attenuate effective binding of
2,6-disubstitued pyridines in general. Such enhanced binding is
critical for a pyridine ligand aiming to emulate the topological
properties of the m-terphenyl framework, especially given a
neutral, rather than anionic, bonding mode. However, topologi-
cally similar, yet neutral variants of the m-terphenyl framework
are appealing to further increase the multielectron redox activity
of a low-coordinate metal center. We believe 2,5-Mes2py and
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Synthesis of [Ag(2,5-Mes2py)2]OTf (1). To a THF solution of
2,5-Mes2py (0.079 g, 0.25 mmol, 2.1 equiv, 3 mL) was added a THF
solution of AgOTf (0.031 g, 0.12 mmol, 3 mL) at room temperature.
The colorless solution was stirred for 12 h, then filtered through Celite,
followed by removal of THF under reduced pressure. Dissolution of the
resulting colorless solid in 2 mL of Et2O followed by filtration and
storage at ꢀ35 °C for 24 h resulted in colorless crystals, which were
collected and dried in vacuo. Yield: 0.073 g, 0.082 mmol, 68%. 1H NMR
(400.1 MHz, C6D6, 20 °C): δ = 8.50 (dd, 2H, 4J = 2 Hz, 5J =1 Hz, o-py),
6.96 (dd, 2H, 3J = 8 Hz, 4J = 2 Hz, p-py), 6.80 (s, 4H, m-Mes), 6.69 (m,
4H, m-Mes), 6.68 (dd, 2H, J = 8 Hz, J = 1 Hz, m-py), 2.17 (s, 6H,
p-CH3-Mes), 2.03 (s, 6H, p-CH3-Mes), 2.02 (s, 12H, o-CH3-Mes), 1.98
(s, 12H, o-CH3-Mes) ppm. 13C{1H} NMR (100.6 MHz, C6D6, 20 °C):
δ = 158.4, 153.0, 139.7, 138.6, 138.3, 138.0, 136.6, 136.3, 136.3, 133.9,
128.8, 124.6, 21.2 (p-CH3-Mes), 21.0 (p-CH3-Mes), 20.8 (o-CH3-Mes),
20.1 (o-CH3-Mes) ppm. 19F{1H} NMR (282.4 MHz, C6D6, 20 °C):
δ = ꢀ77.6 ppm. FTIR (C6D6, KBr windows): 2959 (w, sh), 2924 (m),
2855 (w), 1613 (w), 1552 (vw), 1467 (w, sh), 1455 (w), 1386 (vw),
1377 (vw), 1328 (m), 1291 (m), 1242 (m), 1159 (m), 1053 (w), 1025
(m), 852 (w), 812 (vs), 664 (w), 634 (m) cmꢀ1. Anal. Calcd. for
C47H50 N2F3O3SAg: C, 63.57; H, 5.68; N, 3.16. Found: C, 63.79; H,
5.70; N, 3.08.
Synthesis of trans-Pd(OAc)2(2,5-Mes2py)2 (trans-4). To a
CH2Cl2 solution of Pd(OAc)2 (0.028 g, 0.12 mmol, 3 mL) was added a
CH2Cl2 solution of 2,5-Mes2py (0.079 g, 0.25 mmol, 3 mL, 2.0 equiv).
The yellow reaction mixture was allowed to stir for 16 h, and gradually
darkened in color. The mixture was filtered through Celite and then all
volatile materials were removed in vacuo. Dissolution of the resulting
yellow residue in Et2O (3 mL) followed by filtration and storage
at ꢀ35 °C resulted in yellow crystals, which were collected and dried
in vacuo. Yield: 0.065 g, 0.076 mmol, 61%. 1H NMR (399.9 MHz, C6D6,
20 °C): δ = 10.28 (bd, 2H, 4J = 2 Hz, o-py), 6.98 (s, 4H, m-Mes), 6.80
(dd, 2H, 3J = 8 Hz, 4J = 2 Hz, p-py), 6.70 (s, 4H, m-Mes), 6.49 (d, 2H, 3J =
8 Hz, m-py), 2.47 (s, 6H, OC(O)CH3), 2.13 (s, 6H, p-CH3-Mes), 2.04
(s, 12H, o-CH3-Mes), 2.02 (s, 12H, o-CH3-Mes), 1.83 (s, 6H, p-CH3-
3
5
1
Mes) ppm. 13C{ H} NMR (100.6 MHz, C6D6 , 20 °C): δ = 176.2
(C dO), 160.3, 157.0, 138.9, 137.7, 137.5, 137.0, 136.2, 135.9, 135.5,
133.7, 128.8, 128.7, 125.8, 23.2 (OC(O)CH3), 21.7 ( p-CH3-Mes), 21.1
(p-CH3-Mes), 20.8 (o- CH3-Mes), 20.4 (o-CH3-Mes) ppm. FTIR
(C6D6, KBr windows): ν(CdO) 1642 (vs) cmꢀ1, also 2968 (w), 2918
(w), 2857 (w), 1616 (m), 1551 (w, sh), 1510 (vw), 1474 (m), 1374 (vw,
sh), 1355 (m), 1316 (w, sh), 1305 (s), 1033 (w), 846 (m), 813 (m), 686
(w) cmꢀ1. Anal. Calcd. for C50H56 N 2O4Pd: C, 70.19; H, 6.60; N, 3.28.
Found: C, 69.97; H, 6.59; N, 3.18.
Synthesis of [Cu(2,5-Mes2py)2]OTf (2). To a THF solution of
(μ-C6H6)[CuOTf]2 (0.036 g, 0.072 mmol, 3 mL) was added a THF
solution of 2,5-Mes2py (0.090 g, 0.29 mmol, 4.0 equiv, 3 mL) at room
temperature. The reaction mixture was stirred for 36 h, filtered through
Celite and all volatile materials were removed under reduced pressure.
Dissolution of the resulting colorless residue in a 2:1 toluene/n-pentane
mixture (2 mL) followed by filtration and storage at ꢀ35 °C for 24 h
resulted in colorless crystals, which were collected and dried in vacuo.
Yield: 0.042 g, 0.050 mmol, 69%. 1H NMR (399.9 MHz, C6D6, 20 °C):
δ = 8.19 (bd, 2H, 4J = 2 Hz, o-py), 6.95 (dd, 2H, 3J = 8 Hz, 4J = 2 Hz, p-
py), 6.82 (s, 4H, m-Mes), 6.68 (d, 2H, 3J = 8 Hz, m-py), 6.66 (s, 4H, m-
Mes), 2.19 (s, 6H, p-CH3-Mes), 2.04 (s, 12H, o-CH3-Mes), 2.02 (s, 12H,
o-CH3-Mes), 2.00 (s, 6H, p-CH3-Mes) ppm. 13C{1H} NMR (100.6
MHz, C6D6, 20 °C): δ = 157.8, 151.5, 139.2, 138.7, 138.1, 137.0, 136.7,
136.4, 136.3, 133.9, 128.8, 128.6, 125.1, 21.1 (p-CH3-Mes), 20.9
(p-CH3-Mes), 20.7 (o-CH3-Mes), 20.3 (o-CH3-Mes) ppm. 19F{1H} NMR
(282.4 MHz, C6D6, 20 °C): δ = ꢀ78.3 ppm. FTIR (C6D6, KBr
windows): 2971 (w), 2919 (w), 2863 (w), 1609 (w), 1551 (w, sh),
1468 (m), 1381 (w), 1294 (s), 1238 (s), 1216 (s, sh), 1160 (s), 1028 (s),
847 (m), 793 (w, sh), 639 (s), 574 (w) cmꢀ1. Anal. Calcd. for
C47H50N2F3O3SCu: C, 66.92; H, 5.98; N 3.32. Found: C, 66.71; H,
6.21; N 3.32.
Synthesis of Cu(OTf)2(2,5-Mes2py)2 (cis-3). To a CH2Cl2
solution of Cu(OTf)2 (0.037 g, 0.10 mmol, 3 mL) was added a CH2Cl2
solution of 2,5-Mes2py (0.065 g, 0.21 mmol, 2.0 equiv, 3 mL), which
resulted in a gradual color change from colorless to green over the course
of 12 h. The reaction mixture was stirred for a total of 36 h, then filtered
through Celite and evaporated to dryness under reduced pressure.
Dissolution of the resulting residue in C6H6 (2 mL) followed by
filtration and storage at room temperature for 24 h resulted in green
crystals, which were collected and dried in vacuo. Yield: 0.059 g, 0.059
mmol, 57%. 1H NMR (400.1 MHz, C6D6, 20 °C): δ = 8.17 (bs, 2H, o-
py), 6.85 (bm, 4H, p-py + m-py), 3.79 (bs, 8H, m-2-Mes+ m-5-Mes),
2.23 (bs, 6H, p-CH 3-Mes), 2.05 (bs, 12H, o-CH 3-Mes), 1.80 (bs, 6H,
p-CH 3-Mes), 1.62 (bs, 12H, o-CH 3-Mes) ppm. Evans Method (CDCl3 w/
(Me3Si)2O; 20 °C): μeff = 1.8(1) μB (average of 4 independent
measurements). FTIR (C6D6, KBr windows): 3047 (w, sh), 2976
(w, br), 2924 (w), 2855 (w), 2387 (w, br), 1612 (w), 1554 (vw, sh),
1467 (w), 1381 (w, sh), 1299 (m), 1236 (w), 1219 (w, sh), 1163 (w),
1072 (vw), 1025 (m), 925 (w, br), 850 (w), 813 (m), 662 (vw, sh),
634 (m), 569 (vw) cmꢀ1. Anal. Calcd. for C48H50F6N2O6S2Cu: C,
58.07; H, 5.08; N, 2.82. Found: C, 57.80; H, 5.08; N, 2.57.
Synthesis of [(μ-I)CoI(2,5-Mes2py)]2 (5). To a THF solution of
2,5-Mes2py (0.058 g, 0.18 mmol, 3 mL, 1.0 equiv) was added a THF
solution of CoI2 (0.057 g, 0.18 mmol, 3 mL). The resulting green
reaction mixture was stirred for 36 h, then filtered through Celite and
evaporated to dryness under reduced pressure. The resulting green
residue was washed with n-pentane (3 mL) and dried under reduced
pressure. Dissolution of the resulting green solid in a toluene/n-
pentane/THF mixture (2:2:1, 5 mL total) followed by filtration and
storage at ꢀ35 °C for 24 h resulted in green crystals, which were
collected and dried in vacuo. Yield: 0.085 g, 0.068 mmol, 74%. 1H NMR
(400.1 MHz, C6D6, 20 °C): δ = 7.71 (bs, 6H, o-py + p-py + m-py), 6.82
(bs, 8H, m-2-Mes + m-5-Mes), 2.37 (bs, 12H, o-CH3-Mes), 1.44 (bs,
12H, o-CH3-Mes), ꢀ4.92 (bs, 6H, p-CH3-Mes), ꢀ5.86 (bs, 6H, p-CH3-
Mes) ppm. Evans Method (C6D6 w/ (Me3Si)2O; 20 °C): μeff = 5.9(2)
μB (average of 6 independent measurements). FTIR (C6D6, KBr
windows): 3087 (w, sh), 3060 (w, sh), 3027 (m), 2946 (w, sh), 2919
(m), 2859 (w, sh), 1605 (w), 1497 (s), 1466 (m), 1376 (w), 1077 (w),
1028 (w), 849 (w), 818 (w), 728 (vs), 692 (vs) cmꢀ1. Anal. Calcd. for
C46H50N2Co2I4: C, 43.95; H, 4.01; N, 2.23. Found: C, 44.70; H, 4.64;
N, 1.89.
Synthesis of CoI2(Mes2py)2 (6). To a THF solution of 2,5-
Mes2py (0.033 g, 0.10 mmol, 3 mL, 4.0 equiv) was added a THF solution
of [( μ-I)CoI(2,5-Mes2py)]2 (5, 0.032 g, 0.026 mmol, 3 mL) at room
temperature. The reaction mixture was stirred for 36 h, then filtered
through Celite and evaporated to dryness under reduced pressure.
Dissolution of the resulting green solid in a THF/Et2O mixture (3:1,
4 mL total) followed by filtration and storage at ꢀ35 °C for 24 h resulted
in a small amount of green crystals, which were collected and analyzed by
single-crystal X-ray diffraction. Generation of CoI2(Mes2py)2 for spec-
troscopic analysis was achieved via the addition of 4.0 equiv of 2,5-
Mes2py to [( μ-I)CoI(2,5-Mes2py)]2 (5) in C6D6 solution. The spectral
data reported below were obtained in the presence of excess 2,5-Mes2py.
1H NMR (399.9 MHz, C6D6, 20 °C): δ = 13.65 (bs, 1H, o-py), 9.34 (bs,
1H, m-py), 8.50 (bs, 1H, p-py), 8.24 (bs, 2H, m-Mes) 3.49 (bs, 6H, o-C
H3-Mes), 0.58 (bs, 2H, m-Mes), 0.31 (bs, 6H, o-CH3-Mes), ꢀ6.09 (bs,
12H, p-CH3-Mes + p-CH3-2-Mes) ppm. Satisfactory combustion anal-
ysis and Evans Method magnetic moment determination were not
obtained for this complex because of the necessary presence of excess
2,5-Mes2py.
Synthesis of CoBr2(THF)(2,5-Mes2py) (7). To a THF solution
of 2,5-Mes2py (0.037 g, 0.12 mmol, 3 mL, 1.0 equiv) was added a THF
solution of CoBr2 (0.026 g, 0.12 mmol, 3 mL). The reaction mixture was
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Inorganic Chemistry
ARTICLE
stirred for 36 h, then filtered through Celite and evaporated to dryness
under reduced pressure. The resulting blue residue was washed with n-
pentane (3 mL) and dried under reduced pressure. Dissolution of the
blue residue in a THF/n-pentane mixture (3:1, 2 mL total) and storage
at ꢀ35 °C for 48 h resulted in blue crystals, which were collected and
dried in vacuo. Analysis by 1H NMR spectroscopy indicated the
presence of free THF and resonances for the presumed dimer, [(μ-
Br)CoBr(2,5-Mes2py)]2 (8). Yield: 0.044 g, 0.073 mmol, 62%. 1H NMR
(400.1 MHz, C6D6, 20 °C): δ = 7.97 (bs, 3H, o-py+ p-py+ m-py), 6.89
(bs, 4H, m-Mes), 3.31 (bs, 4H, THF), 2.57 (bs, 6H, o-CH3-Mes), 1.24
(bs, 6H, o-CH3-Mes), 1.13 (bs, 4H, THF), ꢀ3.05 (bs, 3H, p-CH3-Mes),
ꢀ4.72 (bs, 3H, p-CH3-Mes) ppm. Evans Method (C6D6 w/ (Me3Si)2O;
20 °C, on presumed [(μ-Br)CoBr(2,5-Mes2py)]2): μeff = 6.3(2) μB
(average of 4 independent measurements). FTIR (KBr pellet, on
CoBr2(THF)(2,5-Mes2py) solid): 2975 (w), 2950 (w), 2916 (w),
2859 (w, sh), 1610 (m), 1554 (w), 1508 (w), 1473 (m), 1378 (w),
1292 (w), 1228 (vw), 1167 (vw), 1132 (w), 1063 (w), 1018 (m), 919
(w), 865 (m), 855 (m), 735 (w), 666 (w), 619 (w), 572 (m) cmꢀ1. Anal.
Calcd. for C27H33NOCoBr2 (CoBr2(THF)(2,5-Mes2py) solid): C,
53.47; H, 5.49; N, 2.31. Found: C, 53.37; H, 5.43; N, 2.25.
Benzene CꢀH Bond Olefination Reactions. (a). Using trans-
4. In the glovebox, a 25 mL Teflon-capped reaction vessel was charged
with trans-4 (0.042 g, 0.049 mmol, 10 mol %) and a magnetic stir bar.
The vessel was attached to an argon-charged Schlenk line, and acetic
anhydride (45 μL, 0.49 mmol), ethyl acrylate (52 μL, 0.49 mmol), and
2 mL of benzene were added by syringe under an argon blanket. The
reaction mixture was degassed and then backfilled with O2. The reaction
mixture was heated to 60 °C for 6 h then filtered through a pad of Celite.
The filtrate was concentrated under reduced pressure, then washed with
H2O (5 mL). The organic materials were extracted with hexanes (3 ꢁ
5 mL), and then concentrated in vacuo. The residue was purified by flash
chromatography on silica gel using a 95:5 hexanes/EtOAc mixture.
Reported isolated yields are the average of two runs. 1H NMR spectra
and GCMS data for ethyl cinnamate produced by this method are
provided as Supporting Information.
(b). In Situ Mixing of Pd(OAc)2 and 2,5-Mes2py. In the glovebox, a
25 mL Teflon-capped reaction vessel was charged with Pd(OAc)2
(0.014 g, 0.06 mmol, 10 mol %) and 2,5-Mes2py (0.038 g, 0.12 mmol,
20 mol %) and a magnetic stir bar. The vessel was attached to an argon-
charged Schlenk line, and acetic anhydride (57 μL, 0.60 mmol), ethyl
acrylate (64 μL, 0.60 mmol), and 2 mL of benzene were added by syringe
under an argon blanket. The reaction, workup, and isolation were
conducted as described above. Reported isolated yields are the average
of two runs.
Crystallographic Structure Determinations. Single crystal
X-ray structure determinations were carried out at low temperature on
a Bruker P4, Platform or Kappa Diffractometer equipped with a Bruker
APEX detector. All structures were solved by direct methods with SIR
200453 and refined by full-matrix least-squares procedures utilizing
SHELXL-97.54 Crystallographic data collection and refinement infor-
mation is listed in Supporting Information, Table S1. The crystal
structure of 2,5-Mes2py (Supporting Information, Figure S3) suffered
from end-over-end positional disorder of the nitrogen atom and para-
CH group. This disorder was modeled with 50% occupancy of the N and
(CH) in each site and refined.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: jsfig@ucsd.edu.
’ ACKNOWLEDGMENT
We are grateful to the U.S. National Science Foundation for
support through a CAREER Award (CHE-0954710). J.S.F. is a
Cottrell Scholar of Research Corporation and an Alfred P. Sloan
Research Fellow (2011-2013). We thank Dr. Stacey Brydges for
helpful comments concerning ligand design.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures for
b
thermolysis reactions and equilibrium constant determinations,
results of benzene-olefination reactions, disorder model for 2,
5-Mes2py, crystallographic data information, CSD search criteria,
and crystallographic information files (PDF and CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
(23) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long,
G. J.; Power, P. P. Science 2005, 310, 844–847.
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