Coordination and structural properties of encumbering 6-mesityl-2- picolinate complexes

Article abstract of DOI:10.1039/c2dt30585b

In an effort to enforce a sterically hindered environment in transition-metal and main-group 2-picolinate complexes, the synthesis of the encumbering derivative 6-mesityl-2-picolinate (^Mespic) is presented. The coordination and structural properties of ^Mespic are demonstrated with a range of transition-metal and main-group fragments. The 6-position mesityl group of ^Mespic is shown to alter both the primary and secondary coordination spheres of metal centers relative to the ubiquitous and unencumbered parent 2-picolinate anion.

Full text of DOI:10.1039/c2dt30585b

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PAPER
Coordination and structural properties of encumbering 6-mesityl-2-picolinate
complexes†
Patrick W. Smith, Curtis E. Moore, Arnold L. Rheingold and Joshua S. Figueroa*
Received 13th March 2012, Accepted 16th April 2012
DOI: 10.1039/c2dt30585b
In an effort to enforce a sterically hindered environment in transition-metal and main-group 2-picolinate
complexes, the synthesis of the encumbering derivative 6-mesityl-2-picolinate (Mespic) is presented. The
coordination and structural properties of Mespic are demonstrated with a range of transition-metal and
main-group fragments. The 6-position mesityl group of Mespic is shown to alter both the primary and
secondary coordination spheres of metal centers relative to the ubiquitous and unencumbered parent 2-
picolinate anion.
the pioneering studies by Lee, who demonstrated that Ag(pic)2
Introduction
can be employed for the oxidation of aliphatic alcohols to alde-
hydes18 and the oxidative decarboxylation of α-amino acids.19,20
The unsubstituted 2-picolinate anion (pyridine-2-carboxylate) is
a well-utilized ancillary ligand in coordination chemistry. The
longstanding appeal of the 2-picolinate framework is derived
from its ability to form a stable, five-membered chelate ring fea-
turing an anionic binding group. In this respect, 2-picolinate is
sterically and topologically similar to the ubiquitous neutral 2,2′-
bipyridine ligand, yet bears a monoanionic charge. Accordingly,
2-picolinate has been employed in a range of applications,
including the synthesis of metal–organic frameworks (MOFs),1–4
lanthanide chelation/remediation,5–7 and in the case of the tris-
picolinate complex, Cr(pic)3, in a commercial dietary sup-
plement for bioactive chromium ions.8 In addition, its resistance
toward oxidation9 and strong chelation properties have made 2-
picolinate an attractive ligand for the stabilization of transition-
metal ions in high formal oxidation states.10 For this application,
Barton’s use of iron picolinate complexes for catalytic aerobic
alkane oxidation is most notable, as Fe(IV) oxo species are pro-
posed as the critical reactive intermediates.11
In addition to this long-standing coordination chemistry, there
has been a resurgence of interest in 2-picolinate complexes as
reagents in synthesis. This renewed attention has followed
Baran’s recent utilization of silver(II) 2-picolinate (Ag(pic)2) for
the selective, formal C–H bond hydroxylation of guanidinium-
containing rings in aqueous media.12–15 This Ag(II)-mediated
oxidation, which is thought to proceed by two discreet one-elec-
tron transfer processes,16 has also been extended to the C–H
bond arylation of electron-poor nitrogen heterocycles with aryl
boronic acids.17 Organic oxidation reactions by silver(II) build on
Notably, Ag(pic)2 represents one of the few stable and well-
characterized silver(II) complexes.18–26 This fact further high-
lights the ability of the 2-picolinate ligand to stabilize transition
metals in high formal oxidation-states.
Given the broad utilization of the 2-picolinate anion, it is note-
worthy that sterically encumbering variants have not been
widely investigated. Substitution at the 6-position specifically,
which flanks the metal center in 2-picolinate complexes, has
been limited to the 6-methyl27–33 and 6-ethyl34,35 derivatives.36
However, it is reasonable to expect that an encumbering 6-pos-
ition substituent may help foster hindered or low-coordinate
environments for 2-picolinate complexes. Accordingly, in an
effort to couple the favorable coordination properties of 2-picoli-
nate with a more sterically encumbered environment, we report
here the synthesis of 6-mesityl-2-picolinate (Mespic; Mes = 2,4,6-
Me3C6H2) and detail its coordination ability with transition-
metal and main-group fragments. As hoped, the Mespic ligand
can indeed provide a sterically congested coordination environ-
ment and can affect the structural chemistry of metal centers to
which it is bound. This work is an extension of our related inter-
est in the coordination chemistry of encumbering pyridine
ligands that mimic the topological properties of the m-terphenyl
backbone.37
Results and discussion
Scheme 1 shows the synthesis of 6-Mes-2-picolinic acid and
several derivatives for use in metallation reactions. Suzuki–
Miyaura cross-coupling of methyl 6-bromo-2-picolinate with
mesitylboronic acid readily installs a mesityl group on the picoli-
nate framework in good yield (70%). This cross-coupling reac-
tion performs well on scales up to 1.0 g with reasonable
loadings of the palladium catalyst-precursor Pd2(dba)3
(1.4 mol%; 2.8 mol% Pd; dba = dibenzylideneacetone). Base-
Department of Chemistry and Biochemistry, University of California,
San Diego, 9500 Gilman Drive, Mail Code 0358, La Jolla, California
92093-0358, USA. E-mail: jsfig@ucsd.edu
†Electronic supplementary information (ESI) available: Crystallographic
structure determinations and HRMS data (pdf and cif). CCDC
871423–871426. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt30585b
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8031–8038 | 8031

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Fig. 1 Molecular structure of the monomeric portion of Tl(Mespic) (5).
Selected bond distances (Å) and angles (°): Tl–O1 = 2.521(3); Tl–N1 =
2.792(3); N1–Tl–O1 = 61.73(9).
Scheme 1
mediated hydrolysis of the resulting 6-mesityl-2-picolinate
methyl ester (1) with NaOH provides the sodium salt, Na[Mespic]
(2), as a colorless solid. The latter can be obtained in high purity
when worked-up under alkaline conditions and is readily con-
verted to the parent acid, MespicH (3), in a subsequent step by
the addition of aqueous HCl. As expected, the encumbering
mesityl substituent has a solubilizing effect and allows MespicH
(3) to be dissolved in common ethereal and aromatic solvents.
The solid-state IR spectrum (KBr) of MespicH features a strong
absorption band at 1730 cm−1 consistent with the presence of a
carbonyl (CvO) unit. The O–H moiety is found as a broad
signal centered at 2485 cm−1 as is typical for carboxylic acids.
Also present in the IR spectrum of MespicH are moderately
intense bands located at 1610, 1586 and 1447 cm−1, which are
indicative of NvC and CvC stretching modes for pyridyl com-
Fig. 2 Extended solid-state structure of Tl(Mespic) (5). Bond distances
are in Å.
but dissolves readily in both Et2O and benzene. This solubility
profile contrasts with that of Na[Mespic] (2), which is soluble in
H2O, but resists dissolution in low-polarity organic media.
The Mespic framework exhibits versatile coordination proper-
ties to both main-group and transition metal fragments. For
example, a simple 1 : 1 complex between Mespic and Tl(I) is
formed upon treatment of Na[Mespic] (2) with thallium(I) triflate
(TlOTf; OTf = O3SCF3) in a CH2Cl2–H2O mixture (Scheme 1).
The molecular structure of Tl(Mespic) (5) has been determined
by X-ray diffraction (Fig. 1) and exhibits a primary interaction
between one carboxylate-oxygen atom and the Tl center (d(Tl–O1)
= 2.521(3) Å). The Tl–N(1) bond distance in the mononuclear
portion of Tl(Mespic) (5) is considerably longer at 2.792(3) Å.
Whereas low-coordinate complexes of Tl(I) containing mono-
dentate, monoanionic ligands are known,39,40 inspection of the
lattice diagram reveals that Tl(Mespic) (5) adopts an extended 2-
D staircase structure in the solid state (Fig. 2). In the extended
structure, each Tl center makes close contacts with oxygen
atoms from two neighboring Mespic units and is best considered
in an overall four-coordinate, cis-divacant octahedral geometry.
Notably, the intermolecular contact between Tl and the
1
pounds.38 The H NMR spectrum of MespicH in dry C6D6 sol-
ution reveals a broad, downfield signal centered at 10.8 ppm,
consistent with the presence of a carboxylic acid proton engaged
in either intramolecular (to the pyridine nitrogen atom) or inter-
molecular hydrogen-bonding interactions.‡ Treatment of
MespicH (3) with tetrabutylammonium hydroxide ([NBu4]OH) in
a 1 : 1 CH2Cl2–H2O mixture generates the tetrabutylammonium
picolinate salt [NBu4][Mespic] (4), which is insoluble in H2O,
‡The concentration dependence of the carboxylic acid proton chemical
shift in MespicH has not been determined.
8032 | Dalton Trans., 2012, 41, 8031–8038
This journal is © The Royal Society of Chemistry 2012

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Scheme 2
Fig. 3 Molecular structure of [NBu4][CoI2(Mespic)] (6). Selected bond
distances (Å) and angles (°): Co–I1 = 2.5726(5); Co–I2 = 2.5675(5);
Co–O1 = 1.964(2); Co–N1 = 2.062(3); O1–Co–N1 = 83.97(11);
O1–Co–I1 = 112.55(7); O1–Co–I2 = 107.92(7); N1–Co–I1 = 109.72(8);
N1–Co–I2 = 123.32(8); I1–Co–I2 = 115.030(19).
neighboring Mespic carbonyl oxygen atom (i.e. Tl–O2(B)) is sig-
nificantly shorter than the intramolecular Tl–N(1) contact (2.702
(3) vs. 2.792(3)). We believe this short contact likely manifests
from the absence of a ligand trans to the carbonyl oxygen atom
in the extended structure of Tl(Mespic) (5).
In addition to Tl(I) ions, a 1 : 1 complex is also formed
between the Mespic anion and the CoI2 fragment. As shown in
Scheme 1, combination of [NBu4][Mespic] (4) with 1.0 equiv. of
cobalt(II) diiodide in THF solution leads to the paramagnetic salt
[NBu4][CoI2(Mespic)] (6). Structural characterization of [NBu4]-
[CoI2(Mespic)] (6, Fig. 3) revealed a four-coordinate Co center,
which deviates significantly from an idealized tetrahedral geome-
try based on Houser’s τ4 geometry index (τ4 = 0.86 for [NBu4]
[CoI2(Mespic)] (6) vs. 1.0 for idealized Td).41 Indeed, the coordi-
nation geometry of the cobalt center in the [CoI2(Mespic)]− anion
is best described as trigonal monopyramidal with an apical car-
Fig. 4 Molecular structure of Cu(Mespic)2 (7). Selected bond distances
(Å) and angles (°): Cu–O1 = 1.909(2); Cu–O3 = 1.921(2); Cu–N1 =
1.980(2); Cu–N2 = 1.970(2); O1–Cu–O3 = 145.04(9); N1–Cu–N2 =
161.97(9); O1–Cu–N2 = 97.97(9); O3–Cu–N2 = 84.08(9); O1–Cu–N1
= 85.10(9); O3–Cu–N1 = 103.61(9).
In addition to the 1 : 1 Mespic complexes of Tl(I) and the CoI2
fragment, a neutral bis-Mespic complex of Cu(II) can be readily
prepared and its molecular structure further demonstrates the
steric encumbrance posed by the 6-position mesityl group. Treat-
ment of Cu(II) triflate (Cu(OTf)2) with 2.0 equiv. of [NBu4]-
[
Mespic] (4) in Et2O results in the precipitation of the paramag-
netic bis-picolinate complex, Cu(Mespic)2 (7), as a blue solid
(Scheme 2). Evans Method magnetic moment determination in
C6D6 solution (20 °C) gave rise to a μeff value of 1.83(8) μB,
which is close to the spin-only value expected for a d9 Cu(II)
center. Accordingly, crystallographic analysis of Cu(Mespic)2 (7)
revealed a discrete four-coordinate mononuclear Cu center
unperturbed by secondary interactions from neighboring mol-
ecules. Most interestingly, however, the Cu atom within Cu
boxylate oxygen atom and a near-trigonal, NI2 plane (Σ∠(I–N–I)
=
347.06°). This geometry is likely a consequence of both the
small bite angle of the Mespic ligand (83.97(11)°) and minimiz-
ation of steric interference between the iodide ligands and the
encumbering mesityl group. Furthermore, unlike in the 1 : 1
adduct, Tl(Mespic) (5), close intermolecular contacts are not
observed in the solid-state structure of [NBu4][CoI2(Mespic)] (6).
While the large [NBu4]+ cation may obviate such interactions in
the solid state, it is important to note that Evans Method mag-
netic moment determination of [NBu4][CoI2(Mespic)] (6) in
CDCl3 solution resulted in a μeff value of 4.4(1) μB (20 °C). The
magnitude of this magnetic moment is consistent with a mono-
nuclear d7 Co center possessing an S = 3/2 ground state42,43 and
signifies that higher nuclearity species are likely not present to
an appreciable extent in solution at room temperature.
(
Mespic)2 (7) adopts a C2-symmetric, seesaw geometry (Fig. 4).
Metrical parameters in support of this claim for Cu(Mespic)2 (7)
include a bent O1–Cu–O2 angle of 145.04(9)° and a relatively
more linear angle of 161.97(9)° between the trans-oriented N1
and N2 atoms. This seesaw geometry is in sharp contrast to the
undistorted square-planar geometry observed for the unsubsti-
tuted bis-picolinate complex Cu(pic)2.§ The structure of
Cu(pic)2 has been reported by several groups.44–50 Each determi-
nation has shown a trans orientation of the pyridyl units similar
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8031–8038 | 8033

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Fig. 5 Alternative view of the molecular structure of Cu(Mespic)2 (7)
emphasizing the C6–N2–Cu–O1 dihedral angle of 39.04° and the
seesaw Cu coordination geometry.
to that found for Cu(Mespic)2 (7), but with 180° angles between
both the Cu-coordinated nitrogen and oxygen atoms. Accord-
ingly, we believe the distortion observed for Cu(Mespic)2 (7) rela-
tive to Cu(pic)2 results to alleviate steric interactions between
the 6-Mes substituent and the Cu-coordinated oxygen atom. As
shown in Fig. 5, the C6–N2–Cu–O1 dihedral angle in
Cu(Mespic)2 (7) is 39.04°, which affords a 3.223 Å separation
between O1 and the ipso-mesityl carbon atom C7. In a nomin-
ally square-planar configuration, this separation within
Cu(Mespic)2 (7) would undoubtedly be shorter, leading to an
unfavorable close contact between the coordinated O atom and
the mesityl-ring π-face. Thus, as hoped, judicious placement of
an encumbering substituent in the 6-position of the 2-picolinate
framework can force an entatic51 metal-center coordination geo-
metry relative to unhindered picolinate variants.
Fig. 6 Molecular structure of V(Mespic)3 (8). Selected bond distances
(Å) and angles (°): V–O1 = 1.908(2); V–O2 = 1.909(2); V–O3 = 2.052(2);
V–O4 = 2.062(2); V–N1 = 2.158(3); V–N2 = 2.151(3); N1–V–N2 =
159.87(10); O1–V–O2 = 103.60(10); O1–V–O4 = 158.09(10); O2–V–
O3 = 160.66(9); O3–V–O4 = 63.55(9); O3–V–N1 = 105.47(10); O4–
V–N2 = 104.12(11); N1–V–N2 = 159.87(10).
54
55
Cr(pic)3,53 Mn(pic)3 and Co(pic)3 exhibit three, mutually
κ2-N,O-coordinated unsubstituted 2-picolinate ligands. As with
the distortion observed for Cu(Mespic)2 (6), steric pressures
between the 6-position mesityl group and the coordinated
oxygen atoms, or interference between the Mes groups them-
selves, may be reasonably assumed to inhibit a third ligand from
adopting a κ2-N,O coordination mode in V(Mespic)3 (8). Given
that tight κ2-N,O chelation is a hallmark of the 2-picolinate
ligand, it is interesting to speculate that steric destabilization of
this binding mode, as illustrated within the structure of
V(Mespic)3 (8), may lead to enhanced reactivity for an otherwise
kinetically inert metal system.52
Tris-Mespic complexes also exhibit different structural and
coordination properties relative to their less hindered analogues
due to the presence of the encumbering Mes group. As shown in
Scheme 2, the treatment of VCl3(THF)3 with 3.0 equiv. of the
thallium(I) complex, Tl(Mespic) (5), in THF solution results in
the precipitation of TlCl and formation of the paramagnetic tris-
picolinato complex V(Mespic)3 (8). The latter gives rise to a sol-
ution magnetic moment of 2.7(1) μB (C6D6, 20 °C), which is
reflective of an S = 1, d2 vanadium center. Structural characteriz-
ation of V(Mespic)3 (8, Fig. 6) revealed two trans-pyridyl, Mespic
ligands exhibiting the typical κ2-N,O coordination mode.
However, the third Mespic ligand binds the vanadium center in a
κ2-O2 fashion via the carboxylate group and possesses a non-
coordinated pyridyl unit. This 2 : 1 κ2-N,O : κ2-O2 coordination
mode is unique among homoleptic tris-2-picolinate complexes
of the 3d-metals. Indeed, the isostructural complexes V(pic)3,52
Conclusions
The coordination chemistry of the 2-picolinate anion is well
developed and has been exploited for numerous applications. We
have shown here that these established coordination properties
can be further modulated by placement of an encumbering
mesityl group at the 6-position of the 2-picolinate framework.
The 6-mesityl-2-picolinate anion, [Mespic]−, can be synthesized
in two steps from readily available starting materials and several
useful derivatives are prepared by straightforward procedures.
Most importantly, [Mespic]− functions as a ligand to both main-
group and transition-metal fragments. As demonstrated by the
Cu(II) complex, Cu(Mespic)2 (7), steric interference between the
flanking mesityl substituents and a metal-coordinated oxygen
atom can lead to pronounced structural distortions. In this case,
the Cu center in Cu(Mespic)2 (7) adopts a C2-symmetric, seesaw
coordination geometry, rather than a square-planar geometry as
found for the unhindered, parent picolinate complex
Cu(pic)2.44–50 Additionally, the tris-Mespic complex V(Mespic)3
§It is important to note that the unsolvated (ref. 44 and 45) and monohy-
drate (ref. 46–49) forms of Cu(pic)2 feature close intermolecular separ-
ations between the Cu center and carbonyl oxygen atoms from two
neighboring molecules. These intermolecular distances are long (2.770
(8) Å and 2.814(4) Å, respectively), but non-negligible. Similar close
contacts are not observed in the solid-state structure of Cu(Mespic)2 (7).
For comparison, the closest intermolecular contact between the Cu
center in Cu(Mespic)2 (7) and a neighboring carbonyl oxygen atom is
7.390 Å.
8034 | Dalton Trans., 2012, 41, 8031–8038
This journal is © The Royal Society of Chemistry 2012

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(8) features a κ2-O2-coordinated ligand in the solid-state, which
is atypical of 3d-metal M(pic)3 complexes.52–55 This structural
outcome is notable, as it indicates that alleviation of steric press-
ures around the metal coordination sphere outweighs the thermo-
dynamic preference for formation of a third, five-membered
chelate ring. Accordingly, further investigations into the effect of
sterically-induced structural modulation on the reactivity of
these, and other, Mespic-containing complexes are in progress.
achieved either by crystallization from Et2O or by flash chrom-
atography, using 10 : 90 EtOAc : hexanes on basic silica. 1H
NMR (400.6 MHz, CDCl3, 20 °C): δ = 8.10 (dd, 1H, J = 8 Hz,
1 Hz), 7.89 (t, 1H, J = 8 Hz, p-Py), 7.40 (dd, 1H, J = 8 Hz, 1
Hz), 6.91 (s, 1H, m-Mes), 3.99 (s, 3H, CO2CH3), 2.31 (s, 3H, p-
CH3), 2.01 (s, 6H, o-CH3) ppm. 13C{1H} NMR (100.6 MHz,
CDCl3, 20.6 °C): 166.3, 160.5, 148.2, 137.9, 137.2, 137.1,
128.5, 128.4, 123.3, 53.1 (CO2CH3), 21.2 (o-CH3), 20.4 (p-
CH3) ppm. FTIR (KBr pellet, cm−1) ν = 3065 (w), 2960 (m),
2918 (m), 2856 (m), 2730 (w), 1722 (s, νCO), 1677 (w), 1611
(m), 1584, 1565 (m), 1456, 1379 (m), 1272 (m), 1235, 1083,
1055, 991, 855 (m). HRMS (ESI) m/z Found: 256.1332
[M + H]+.
Experimental section
General considerations
Unless otherwise stated, all manipulations were carried out under
an atmosphere of dry dinitrogen using standard Schlenk and glo-
vebox techniques. Solvents were dried and deoxygenated accord-
ing to standard procedures.56 Reagent grade starting materials
were purchased from commercial sources and were used as
received or purified by standard procedures.57 Benzene-d6 (Cam-
bridge Isotope Laboratories) was degassed and stored over 4 Å
molecular sieves for 2 d prior to use. Celite 405 (Fisher Scien-
tific) was dried under vacuum (24 h) at a temperature above
Synthesis of sodium 6-mesityl-2-picolinate (Na[Mespic], 2)
To an Et2O solution of methyl 6-mesitylpyridine-2-carboxylate
(
MespicMe (1); 0.810 g, 3.17 mmol, 90 mL) was added 8 mL of
a 2 M methanolic solution of NaOH (16.0 mmol, 5.0 equiv.).
The reaction mixture was stirred for 1 h, resulting in the for-
mation of a colorless precipitate. The reaction mixture was
diluted with Et2O (100 mL), filtered, and the resulting solid
washed with Et2O (1 × 50 mL) and n-pentane (1 × 50 mL). The
resulting colorless solid was collected and dried in vacuo over-
1
250 °C and stored in the glovebox prior to use. Solution H and
13C{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-protio resonances of 7.16 ppm (1H) and
128.06 ppm (13C) for benzene-d6, and 7.26 ppm (1H) and
77.16 ppm (13C) for CDCl3. FTIR spectra were recorded on a
Thermo-Nicolet iS10 FTIR spectrometer. Samples were prepared
as KBr pellets. The following abbreviations were used for the
intensities and characteristics of important IR absorption bands:
vs = very strong, s = strong, m = medium, w = weak, vw = very
weak; b = broad, vb = very broad, sh = shoulder. High resolution
mass spectrometry (HRMS) was performed using an Agilent
6230 ESI-TOFMS instrument in either positive or negative ion
mode as necessary. Combustion analyses were performed by
Robertson Microlit Laboratories of Ledgewood, NJ (USA).
1
night. Yield: 0.812 g, 2.92 mmol, 92%. H NMR (400.6 MHz,
D2O, 20 °C): δ = 7.94 (t, 1H, J = 8 Hz) 7.78 (d, 1H, J = 8 Hz,
p-Py), 7.29 (d, 1H, J = 8 Hz), 7.01 (s, 1H, m-Mes), 2.28 (s, 3H,
p-CH3), 1.92 (s, 6H, o-CH3) ppm. 13C{1H} NMR (100.6 MHz,
D2O with methanol-d4 internal standard, 20 °C): 175.0, 160.1,
140.0, 139.8, 137.8, 127.9, 123.1, 21.6 (o-CH3), 20.6 (p-CH3)
ppm. FTIR (KBr pellet, cm−1) ν = 2973 (m), 2942 (m), 2919
(m), 2857 (w), 2729 (w), 1621 (s, νCO), 1578 (s, νCO), 1562,
1449 (m), 1430 (m), 1377, 1231 (w), 1172 (w), 1081 (w), 1059
(w), 995 (w), 867 (m), 784, 749 (m), 659 (m). HRMS (ESI) m/z
Found: 240.1031 [M]−.
Synthesis of 6-mesityl-2-picolinic acid (MespicH, 3)
Synthesis of methyl 6-mesityl-2-picolinate (MespicMe, 1)
To an acetone slurry of sodium 6-mesitylpyridine-2-carboxylate
(Na[Mespic] (2); 0.522 g, 1.88 mmol) was added aqueous HCl
(12 M, 0.156 mL, 1.88 mmol, 1.0 equiv.) via syringe. The
mixture was placed in an ultrasonic bath for 40 min, which
resulted in full dissolution of all precipitates. The resulting sol-
ution was then filtered through Celite and all volatiles were
removed from the filtrate in vacuo to yield a colorless solid.
Yield: 0.330 g, 1.29 mmol, 69%. 1H NMR (400.6 MHz, CDCl3,
20 °C): δ = 10.76 (s, 1H, CO2H), 7.87 (dd, 1H, J = 8 Hz, 1 Hz),
6.91 (t, 1H, J = 8 Hz, p-Py), 6.79 (s, 1H, m-Mes), 6.66 (dd, 1H,
J = 8 Hz, 1 Hz), 2.16 (s, 3H, p-CH3), 1.79 (s, 6H, o-CH3) ppm.
13C{1H} NMR (100.6 MHz, CDCl3, 20 °C): 163.6, 158.9,
146.6, 138.5, 138.3, 135.7, 121.6, 21.1 (o-CH3), 20.2 (p-CH3)
ppm. FTIR (KBr pellet, cm−1) ν = 3088 (m), 2922, 2855 (m),
2731 (m), 2485 (m, vb ν(OH)), 1730 (s, νCO), 1684 (w), 1625
(m), 1610 (m), 1587 (m), 1515 (w), 1448 (m), 1290 (m), 1261
(m), 1226, 1175 (m), 2085 (m), 1032 (w), 1002 (m), 842 (m),
777 (m), 709 (m). HRMS (ESI) m/z Found: 242.1175. [M + H]+.
In the glovebox, tris(dibenzylideneacetone)dipalladium(0)-
(Pd2(dba)3; 0.060 g, 0.066 mmol, 1.4 mol%), tricyclohexylpho-
sphine (PCy3; 0.040 g, 0.144 mmol, 3.1 mol%), and K3PO4
(1.85 g, 8.19 mmol, 1.8 equiv.) were placed in a 100 mL airtight
reaction vessel. On the Schlenk line, under an argon blanket,
methyl 6-bromopyridine-2-carboxylate (1.00 g, 4.62 mmol),
mesitylboronic acid (MesB(OH)2; 1.00 g, 6.1 mmol, 1.3 equiv.)
and toluene (20 mL) were added to the reaction vessel and
sealed. The reaction mixture was heated in an oil bath at 100 °C
for 24 h. Upon cooling, 50 mL of a saturated aqueous sodium
bicarbonate solution and 40 mL of diethyl ether were added to
the mixture and stirred for 1 h. The organic and aqueous layers
were separated, and the aqueous layer extracted with Et2O (2 ×
100 mL). The combined organic layers were dried over mag-
nesium sulfate and evaporated to dryness, which yielded a color-
less solid. Yield: 0.810 g, 3.19 mmol, 70%. This material can be
used without further purification. However, purification can be
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 8031–8038 | 8035

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Synthesis of terabutylammonium 6-mesityl-2-picolinate ([NBu4]-
Mespic], 4)
0.082 mmol, 1.0 equiv., 2 mL). The mixture was stirred for
30 min while changing in color from green to teal. The reaction
mixture was then filtered through Celite, layered with Et2O and
cooled to −35 °C for 12 h to produce a teal solid that was col-
lected and dried in vacuo. Yield: 0.049 g, 0.061 mmol, 74%.
Single-crystal X-ray diffraction quality crystals of NBu4[CoI2Ar-
MesPic]·C6H6 were grown from diffusion of n-pentane into a
saturated benzene solution at room temperature. 1H NMR
(400.6 MHz, CDCl3, 20 °C): δ = 35.9 (s), 11.7 (s), 4.8 (s), 3.5
(s), 2.2 (s), −0.45 (s), −2.6 (s), −5.2 (s), −9.0 (s), −22.7 (s)
ppm. FTIR (KBr pellet, cm−1) ν = 2958, 2873 (m), 2732 (w),
1655 (s, νCO), 1610 (m), 1593, 1564 (w), 1488 (m), 1460 (m),
1377 (m), 1341 (m), 1242 (m), 1157 (m), 1080 (m), 1060 (m),
1021 (m), 880 (w), 866 (w), 848 (w), 780 (m), 760 (m), 740
(m), 675 (w). μeff (Evans method, 500 MHz, CDCl3 with
O(SiMe3)2, 20 °C) = 4.4 ( 0.1) μB (average of 5 independent
measurements) Anal. calcd for CoI2C31N2O2H50: C, 46.81; H,
6.34; N, 3.52. Found: C, 46.24; H, 5.55; N, 3.26.
[
To a slurry of MespicH (3) in H2O (0.300 g, 1.24 mmol, 4 mL)
was added [NBu4]OH·30H2O (1.50 g, 1.87 mmol, 1.5 equiv.),
followed by 10 mL of CH2Cl2. The reaction mixture was stirred
for 1 h. The organic and aqueous layers were then separated and
the aqueous layer was washed with CH2Cl2 (3 × 20 mL). The
organic layers were combined and dried under reduced pressure.
The resulting oily residue was placed in a vacuum desiccator
with phosphorus pentoxide for 2 d. The resulting solid was then
transferred to a glovebox, slurried in Et2O and then dried
thoroughly in vacuo. Recrystallization from a THF–Et2O
mixture at −35 °C produced a colorless solid, which was col-
1
lected and dried in vacuo. Yield: 0.382, 0.794 mmol 64%. H
NMR (400.6 MHz, C6D6, 20 °C): δ = 8.55 (dd, 1H, J = 8 Hz,
1 Hz), 7.37 (t, 1H, J = 8 Hz), 6.84 (s, 1H, m-Mes), 6.82 (d, 1H,
J = 1 Hz), 3.48 (m, 8H, NBu4 NCH2), 2.21 (s, 3H, p-Mes CH3),
2.13 (s, 6H, o-Mes CH3), 1.60 (m, 8H, NBu4 methylene), 1.34
(m, 8H, NBu4 methylene), 0.95 (t, 12H, J = 7.2 Hz, NBu4 CH3)
ppm. 13C{1H} NMR (100.6 MHz, CDCl3, 20 °C): 169.36,
161.9, 157.9, 140.3, 136.1, 135.4, 128.5, 123.0, 122.7, 58.5,
24.2, 21.2, 20.8, 20.3, 14.2 ppm. FTIR (KBr pellet, cm−1) ν =
2956, 2870, 2372 (w), 1619 (s, νCO), 1574 (s, νCO), 1491 (m),
1480 (m), 1394 (m), 1949, 1280 (w), 1229 (w), 1165 (m), 1150
(m), 1074 (m), 1058 (m), 1030 (m), 989 (m), 885 (m), 854 (m),
738, 654 (m). HRMS (ESI) m/z Found: 240.1032 [M]−.
Synthesis of Cu(Mespic)2 (7)
To an Et2O solution of CuOTf2 (0.026 g, 0.072 mmol, 3 mL)
was added an Et2O slurry of [NBu4][Mespic] (4; 0.070 g,
0.14 mmol, 2.0 equiv., 5 mL). The reaction mixture was stirred
for 36 h, resulting in the formation of a blue precipitate. The
reaction mixture was filtered and the resulting solid extracted
with THF (2 mL). Layering this solution with n-pentane, fol-
lowed by storage at −35 °C for 12 h produced single-crystal X-
ray diffraction quality crystals. Yield: 0.030 g, 0.054 mmol,
Synthesis of thallium(I) 6-mesityl-2-picolinate (Tl(Mespic), 5)
1
To an aqueous solution of Na[Mespic] (2, 0.842 g, 3.20 mmol,
4 mL) was added an aqueous solution of thallium(I) triflate
(TlOTf; 1.200 g, 3.39 mmol, 1.05 equiv., 4 mL), followed by
40 mL of CH2Cl2. The mixture was stirred for 1 h, after which
the organic and aqueous layers were separated. The aqueous
layer was then extracted with CH2Cl2 (3 × 100 mL). The com-
bined organic layers were evaporated to dryness and the resulting
residue dissolved in 10 mL of CH2Cl2. 50 mL of n-pentane were
then added, resulting in the precipitation a colorless solid, which
was collected and dried in vacuo. Yield: 1.10 g, 2.47 mmol,
77%. Single-crystal X-ray diffraction quality crystals were grown
75%. H NMR (400.6 MHz, CDCl3, 20 °C): δ = 9.1 (s), 6.8 (s),
3.3 (s, very broad), 1.7 (s) ppm. FTIR (KBr pellet, cm−1) ν =
3073 (w), 2920 (m), 2857 (m), 2734 (w), 1674 (s, νCO), 1599,
1563 (m), 1460, 1396 (m), 1377 (m), 1330, 1301 (m), 1279 (m),
1242, 1179 (m), 1148, 1077 (m), 1029 (m), 871 (m), 789, 767
(m), 861 (m), 595 (w). μeff (Evans method, 500 MHz, CDCl3
with O(SiMe3)2, 20 °C) = 1.83 ( 0.08) μB (average of 4 indepen-
dent measurements). Anal. calcd for CuC30N2O4H28: C, 66.22;
H, 5.19; N, 5.15. Found: C, 65.88; H, 5.15; N, 5.01.
Synthesis of V(ArMesPic)3 (8)
1
from a saturated toluene–CHCl3 (10 : 1) mixture at −35 °C. H
58
NMR (400.6 MHz, CDCl3, 20 °C): δ = 7.93 (d, 1H, J = 7 Hz,
1 Hz), 7.83 (t, 1H, J = 7 Hz, p-Py), 7.30 (d, 1H, J = 7 Hz), 6.96
(s, 1H, m-Mes), 2.34 (s, 3H, p-CH3), 2.00 (s, 6H, o-CH3) ppm.
13C{1H} NMR (100.6 MHz, CDCl3, 20 °C): 172.7, 157.4,
155.0, 138.5, 138.2, 136.1, 135.9, 128.7, 126.7, 123.8, 21.3
(o-CH3), 20.5 (p-CH3) ppm. FTIR (KBr pellet, cm−1) ν = 3065
(w), 2948 (m), 2917 (m), 2856 (w), 2732 (w), 1596 (s, νCO),
1570 (s, νCO), 1488 (w), 1453 (m), 1396 (m), 1378 (m), 1360,
1280 (w), 1236 (m), 1172 (w), 1150 (w), 1080 (m), 1057 (m),
1033 (w), 998 (m), 852 (m), 840 (m), 776, 753 (m), 734 (m),
660 (m). Anal. calcd for TlC15NO2H14: C, 40.52; H, 3.17; N,
3.15. Found: C, 40.35; H, 3.01; N, 3.23.
To a THF solution of VCl3(THF)3 (0.042 g, 0.112 mmol,
6 mL) was added a THF solution of Tl(Mespic) (5, 0.150 g,
0.336 mmol, 3.0 equiv., 4 mL). The reaction mixture was stirred
for 2 h, resulting in the formation of a colorless precipitate. The
mixture was then filtered through Celite and all volatile materials
were removed under reduced pressure. The resulting green
residue was dissolved in toluene, layered with pentane and
stored at −35 °C overnight to afford green crystals. Yield:
0.050 mg, 0.063 mmol, 56%. Single-crystal X-ray diffraction
quality crystals of V(Mespic)3·THF were grown by diffusion of n-
1
pentane into a saturated THF solution. H NMR (400.6 MHz,
C6D6, 20 °C): δ = 24.1, 19.2, 10.7, 8.2, 6.7 (s), 3.5 (s, very
broad), 3.1 (s), −6.6 (s), −18.8 (s) ppm. FTIR (KBr pellet,
cm−1) ν = 3069 (w), 3003 (m), 2956 (m), 2920 (m), 2858 (m),
2734 (w), 1707 (s, νCO), 1690 (s, νCO), 1610 (m), 1599 (m),
1564 (m), 1516 (m), 1474 (m), 1460 (m), 1420 (m), 1382 (m),
1319, 1300 (m), 1273 (m), 1229 (m), 1162 (m), 1146, 1084 (m),
Synthesis of [NBu4][CoI2(Mespic)] (6)
To a THF solution of CoI2 (0.026 g, 0.082 mmol, 3 mL) was
added
a
THF solution of [NBu4][Mespic] (4; 0.040 g,
8036 | Dalton Trans., 2012, 41, 8031–8038
This journal is © The Royal Society of Chemistry 2012

View Online
Table 1 Crystallographic data and refinement information
Tl(Mespic) (5) [NBu4][CoI2(Mespic)] (6·C6H6)
C6H6 Cu(Mespic)2 (7)
V(Mespic)3·THF (8·THF)
Formula
Crystal System
Space Group
a/Å
b/Å
c/Å
C
15H14NO2Tl
C37H56CoI2N2O2
Triclinic
P1
C30H28CuN2O4
Orthorhombic
Pca21
19.0917(14)
7.7213(6)
17.8169(13)
90
90
90
2626.4(3)
4
C49H50N3O7V
Monoclinic
P21/n
11.7453(16)
12.1951(15)
29.336(4)
90
96.980(7)
90
4170.9(9)
4
Monoclinic
P21/n
5.4362(3)
14.4583(7)
17.0112(9)
90
94.3580(10)
90
1333.18(12)
4
ˉ
9.7875(6)
11.8633(7)
18.2575(10)
101.7360(10)
102.8450(10)
103.3080(10)
1937.6(2)
2
α (°)
β (°)
γ (°)
V/Å3
Z
Radiation, λ/Å
ρ (calcd) g cm−3
μ (Mo Kα), mm−1
Temperature/K
θ max (°)
data/parameters
R1
Mo Kα, 0.71073
2.215
12.111
100(2)
25.46
2480/175
0.0200
0.0429
Mo Kα, 0.71073
1.497
2.068
100(2)
25.45
7117/404
0.0320
0.0648
Mo Kα, 0.71073
1.376
0.870
100(2)
25.47
4126/340
0.0294
0.0688
Mo Kα, 0.71073
1.344
0.295
100(2)
25.82
7987/550
0.0620
0.0931
wR2
GOF
1.031
1.036
1.035
1.022
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1056 (m), 1016 (m), 990 (w), 879 (m), 847 (m), 768, 667 (m),
660 (m). μeff (Evans method, 500 MHz, C6D6 with O(SiMe3)2,
20 °C) = 2.7 ( 0.1) μB (average of 5 independent measure-
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ometer equipped with a Bruker APEX detector. All structures
were solved by direct methods with SIR 200459 and refined by
full-matrix least-squares procedures utilizing SHELXL-97.60
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Acknowledgements
We thank the California Institute of Telecommunications and
Information Technology (CalIT2) for an undergraduate research
scholarship to P.W.S. We also gratefully acknowledge the U.S.
National Science Foundation for support through a CAREER
Award (CHE-0954710) and NMR instrumentation grants to the
UCSD Department of Chemistry and Biochemistry
(CHE-0741968 and CHE-9709183). J.S.F. is an Alfred P. Sloan
Research Fellow (2011–2013) and a Cottrell Scholar of Research
Corporation (2010–2012).
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8038 | Dalton Trans., 2012, 41, 8031–8038
This journal is © The Royal Society of Chemistry 2012