Fine-Tuning Metal and Ligand-Centered Redox Potentials of Homoleptic Bis-Terpyridine Complexes with 4′-Aryl Substituents

Article abstract of DOI:10.1021/acs.inorgchem.1c01233

Homoleptic transition-metal complexes of 2,2':6′,2″-terpyridine (terpy) and substituted derivatives of the form [M(R-terpy)2]2+ display a wide range of redox potentials that correlate well to the Hammett parameter of the terpy substituents. Less is known about the impact of incorporating a phenyl spacer between the functional group responsible for controlling the electron density of terpy and how that translates to metal complexes of the form [M(4′-aryl-terpy)2]2+, where M = Mn, Fe, Co, Ni, and Zn. Herein, we report our studies on these complexes revealed a good correlation of redox potentials of both metal- and ligand-centered events with the Hammett parameters of the aryl substituents, regardless of aryl-substitution pattern (i.e., the presence of multiple functional groups, combinations of withdrawing and donating functional groups). The phenyl spacer results in 60-80% attenuation of electron density as compared to the 4′-substituted terpy analogue, depending on the metal and redox couple analyzed. Density functional theory calculations performed on a simple model system revealed a strong correlation between the Hammett parameters and lowest unoccupied molecular orbital energies of the corresponding substituted pyridine models, thus serving as an inexpensive predictive tool when coupled with electrochemical data. Overall, these data suggest that such ligand modifications may be used in combination with previous approaches to further fine-tune the redox potentials of homoleptic transition-metal complexes, which may have applications in photochemical and electrochemical catalytic processes.

Full text of DOI:10.1021/acs.inorgchem.1c01233

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Article
Fine-Tuning Metal and Ligand-Centered Redox Potentials of
Homoleptic Bis-Terpyridine Complexes with 4′-Aryl Substituents
John C. Dickenson, MacKenzie E. Haley, Jacob T. Hyde, Zachary M. Reid, Travis J. Tarring,
Diana A. Iovan, and Daniel P. Harrison*
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ABSTRACT: Homoleptic transition-metal complexes of 2,2’:6′,2″-
terpyridine (terpy) and substituted derivatives of the form [M(R-
terpy)2]2+ display a wide range of redox potentials that correlate
well to the Hammett parameter of the terpy substituents. Less is
known about the impact of incorporating a phenyl spacer between
the functional group responsible for controlling the electron density
of terpy and how that translates to metal complexes of the form
[M(4′-aryl-terpy)2]2+, where M = Mn, Fe, Co, Ni, and Zn. Herein,
we report our studies on these complexes revealed a good
correlation of redox potentials of both metal- and ligand-centered
events with the Hammett parameters of the aryl substituents,
regardless of aryl-substitution pattern (i.e., the presence of multiple
functional groups, combinations of withdrawing and donating
functional groups). The phenyl spacer results in 60−80%
attenuation of electron density as compared to the 4′-substituted terpy analogue, depending on the metal and redox couple
analyzed. Density functional theory calculations performed on a simple model system revealed a strong correlation between the
Hammett parameters and lowest unoccupied molecular orbital energies of the corresponding substituted pyridine models, thus
serving as an inexpensive predictive tool when coupled with electrochemical data. Overall, these data suggest that such ligand
modifications may be used in combination with previous approaches to further fine-tune the redox potentials of homoleptic
transition-metal complexes, which may have applications in photochemical and electrochemical catalytic processes.
for a series of 4′-substituted terpy-supported iron, cobalt, and
manganese complexes of the form [M(4′-R-terpy)2]2+.4,18,19
Similar behavior has been observed with 4′-R-terpy complexes
of ruthenium and osmium.29
INTRODUCTION
■
Terpyridine (terpy, 2,2′:6′,2″-terpyridine) ligands have been
widely utilized as platforms for transition-metal complexes,
because of their versatility in controlling the electronic and
steric properties of metal complexes and have found numerous
applications in fields such as catalysis (e.g., electrocatalytic
proton, CO2, and oxygen reduction),1−7 photochemistry,8−12
and supramolecular assemblies.13−16 Fontecave, Slattery, and
Similarly, the correlation between the Hammett parameter
and metal and ligand reduction potentials for phenyl-
substituted corroles and porphyrins has been well-docu-
mented.25,26,30,31 Incorporating substituents in these latter
systems impacted the properties of interest to a various extent,
depending on the overall metal-coordination environment
(e.g., coordination number, structural changes, or chemical
events upon oxidation and/or reduction) and degree of
electronic communication with the metal center or ligand,
respectively.
̈
Sjodin have synthesized numerous bis-terpy compounds that
incorporate functional groups with a variety of π-acidities
directly onto the terpyridine ligand in order to investigate their
impact on the electrochemical and other properties of these
homoleptic bis-terpyridine complexes of the first-row transition
metals of cobalt, iron, and manganese (Figure 1).4,17−19
Although there are a handful of examples correlating
Hammett parameters of substituted organic compounds to
reduction potentials,20−26 with the exception of ferrocene
Received: April 21, 2021
Published: June 23, 2021
derivatives,27,28 Slattery’s and Sjodin’s studies are among the
̈
few describing such relationships with metal- and/or ligand-
centered reduction potentials of first-row transition-metal
complexes. In these cases, Hammett parameters were
correlated with MIII/II and ligand-centered reduction potentials
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Inspired by this precedent, and given the importance of
controlling redox potentials in catalysis, we sought to
determine if further fine-tuning of the redox potentials of
[M(L)2]2+ can be accomplished via incorporation of 4′-aryl
substituents onto terpyridine ligands. 4′-aryl-terpy (L) ligands
were attractive candidates for this initiative because of their
ease of synthesis. In addition, these ligands provide a means to
determine the impact of a phenyl spacer placed between the
terpyridine scaffold and the functional group(s) responsible for
tuning the electronic properties of both the ligand itself and
the electrochemical potentials of the coordinated metal.
Herein, the synthesis and electrochemical characterization of
a series of homoleptic metal (M = Mn, Fe, Co, Ni, Zn)
complexes of 4′-aryl-substituted terpyridine ligands featuring
aryl units decorated with a range of electron-withdrawing/
electron-donating groups is described. Electrochemical meas-
urements revealed good linear trends of metal- and ligand-
based reduction potentials, with respect to Hammett
Figure 1. Comparison of previous work to current work. (Left)
̈
Slattery and Sjodin have reported the influence of a variety of
functional groups at the 4′-R group, while Fontecave’s work primarily
studied functionalization of terminal pyridine rings in conjunction
with the 4′-position. (Right) Here, we focus on the influence of the
phenyl spacer at the central pyridine.
b
Figure 2. (A) Synthetic scheme and ligands utilized in this work. [Superscript a represents 2 new ligands presented here. represents 20 new
complexes.] (B) Single-crystal structure of [Co(3-CF3,4-OMe)2][PF6]2 at the 50% thermal ellipsoid level. Hexafluorophosphate anions and
1
1
protons are omitted for clarity. (C) (Left) H NMR spectrum of [Co(3-CF3,4-OMe)2][PF6]2 in blue; (right) maroon H NMR spectrum
corresponding to the [Co(3-CF3,4-OMe)2][PF6]2[BF4] generated in situ upon the addition of AgBF4, while the blue-green spectrum corresponds
to uncoordinated ligand in the same NMR solvent. All NMRs were collected in acetone-d6.
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Figure 3. Cyclic voltammograms of the two characteristic behaviors observed for the [Co(L)2]2+ series. All CVs were collected in MeCN at a scan
rate of 100 mV/s with 0.1 M TBAPF6 and are referenced vs [FeCp2]+/FeCp2, using a 3-mm-diameter glassy carbon electrode with X-ray-quality
crystalline materials. Each CV was initiated near −0.5 V and scanned in the anodic direction until an appropriate switching potential. 2.5 further
cycles are presented, ending at the cathodic switching potential. Panel (A) is 1.1 mM [Co(3,5-CF3)2]2+. The blue and red CVs in panel (B) is 1.9
mM [Co(3,5-Br)2]2+. Note that the gray arrows indicate diminishing current with successive scans in the red trace. The purple and cyan CVs in
panel (B) are 1.9 mM [Co(3,4,5-F)2]2+.
parameters of the aryl substituents. Furthermore, the narrow
redox potential window noted for a series of such Co
complexes reflects the possibility of fine-tuning redox
potentials via ligand modifications, expanding upon the
currently available substituted-terpy redox control.
Paramagnetic cobalt complexes produced 1H NMR
resonances over a range of 5−100 ppm (see Figure 2B, as
well as Figures S39−S64 in the Supporting Information),
displaying the expected number of ligand resonances. Greater
broadening is observed for more downfield-shifted signals,
whereas sharp methoxy resonances appear near their normal
diamagnetic chemical shifts. The addition of excess AgBF4 to
the deep red-orange solutions of [Co(L)2]2+ in deuterated
solvents affords yellow solutions of diamagnetic species (as
RESULTS AND DISCUSSION
■
Synthesis and NMR, XRD, and CV Characterization of
Metal Complexes. To investigate the effect of 4′-aryl
substituents on the redox potential of bis-terpyridine
complexes, a series of electron-withdrawing (bromo-, fluoro-,
trifluoromethyl-) and electron-donating (methoxy-) groups
were systematically incorporated throughout the aryl ring at
the 4′-position of the terpyridine ligand: ortho- (2-CF3, 2,6-F,
2-OMe), meta- (3-CF3, 3,5-CF3, 3,5-F, 3-OMe, 3,5-OMe, 3-
Br, 3,5-Br), para- (4-CF3, 4-OMe), and mixed positions (3-
CF3, 4-OMe, 3,4,5-F, 2,4-OMe, 2,4,6-OMe, 3,4,5-OMe) (see
Figures S1 and S2 in the Supporting Information). The ligands
were rapidly accessed upon condensation of 2-acetylpyridine
with the appropriately substituted benzaldehyde (Figure 2A,
yields between 18% and 66% with an average of 45% for 17
terpyridines, and an average mass yield of 4.4 g). Metalation
with divalent metal sources (halides, sulfate), followed by
treatment with ammonium hexafluorophosphate afforded the
corresponding [M(L)2][PF6]2 metal complexes in good yields
(Figure 2A, yields between 23% and 96% with an average of
73% for 32 complexes). Diethyl ether vapor diffusion into
solutions of acetonitrile or acetone of most of these metal
complexes afforded single crystals suitable for X-ray diffraction
to further corroborate their structural identity (see Figures 2B,
as well as Figures S4−S10 and S13−S32 in the Supporting
Information). All of these pseudo-octahedral complexes display
bond lengths and torqued aryl rings relative to the planar-
coordinated terpy consistent with previously reported [M-
(terpy)2]2+ and [M(L)2][PF6]2, with the exception of four
meta-substituted structures (i.e., [Co(3,5-F)2][PF6]2, [Co-
(3,4,5-F)2][PF6]2, [Co(3-OMe)2][PF6]2, and [Ni(3-OMe)2]-
[PF6]2), which displayed deviation from orthogonality between
the two distorted terpy planes of up to 7° (see Tables S11 and
S12 in the Supporting Information).32−39
1
evidenced by H NMR spectroscopy) assigned as [Co(L)2]-
[PF6]2[BF4], further validating the identity of the CoII species
(see Figure 2C, as well as Figures S39−S63). This NMR
behavior is consistent with previous characterization of similar
[CoII(L)2]2+ and [CoIII(L)2]3+ species.40,41 Nickel complexes
1
produced similar paramagnetic H NMR resonances, except
for an unobserved proton signal likely absent because of
extreme baseline broadening (see Figure S68 in the Supporting
Information). Manganese complexes revealed proton reso-
nances in the normal aromatic region; however, broadening
and overlap prevented meaningful characterization (see Figure
S65 in the Supporting Information). In contrast, metalation of
the prepared ligands with iron and zinc afforded diamagnetic
1
complexes as evidenced by H and 13C NMR (Figures S66,
S67, and S69−S71 in the Supporting Information). Sharp 19F
signals are observed in all complexes featuring fluoro- or
trifluoromethyl-substituted ligands, regardless of paramagnet-
ism, in addition to the signals corresponding to PF6−
counterions (see Figures S39−S64 (for Co), Figure S65 (for
Mn), Figures S66 and S67 (for Fe), Figure S68 (for Ni), and
Figures S69−S71 (for Zn)).
Cyclic voltammetry (CV) was used to explore the redox
behavior of [M(L)2]2+ complexes. CV data were collected for
each species in anhydrous acetonitrile, MeCN, with 0.1 M
tetra-n-butylammonium hexafluorophosphate, TBAPF6, sup-
porting electrolyte and 0.1 V/s scan rates. All potentials were
internally referenced versus the ferrocene/ferrocenium,
[FeCp2]+/FeCp2, redox couple with a glassy carbon electrode.
Metals with accessible MIII/II (e.g., Mn, Fe, Co, and Ni)
couples were generally well-behaved reversible redox couples
(see Figures S3−S8 and S10−S12), with the exception of Mn
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complexes (Figure S9). All Mn compounds displayed peak-to-
peak voltages (ΔEp = |Ep,a − Ep,c|) > 100 mV, similar to related
[Mn(4′-R-terpy)2]2+ complexes, which has been postulated to
be a consequence of an expected Jahn−Teller distortion
associated with oxidation to MnIII.19,42 Two well-behaved
ligand-centered reductions are observed when M = Mn, Fe, Ni,
and Zn. Examination of the [Co(L)2]2+ complexes CV data
suggests that they can be classified into two categories,
dependent upon their respective electrochemical behaviors,
specifically the number of reversible redox events they
displayed (see Figures S3 and S4 and Figures S5−S8,
respectively).
In the first class, the majority of [Co(L)2]2+ complexes
display four reversible redox couples: one metal oxidation
event (CoIII/II), one metal reduction event (CoII/I), and two
reversible ligand-centered reductions (see Figure 3A, as well as
Figures S3 and S4). Analysis of these reduction potentials
reveals a redox span of 0.12, 0.16, 0.29, and 0.36 V for the
CoIII/II, CoII/I, Lπ0/−1, and Lπ−1/−2, respectively.
Figure 4. Structures of 4′-NEt2-terpy, 4′-SO2Me-terpy, and 4′-OMe-
terpy previously investigated by Slattery.18
remarkable that, despite the phenyl spacer, ΔE1/2(CoIII/II) =
0.077 V for the aryl series (ΔE1/2(CoIII/II) = E1/2([Co(3,4,5-
F)2]2+) − E1/2([Co(4-OMe)2]2+)) is ∼40% relative to that
observed by Slattery for complexes of similar donor capacities
in the four alkyl series (4′-R ΔE1/2(CoIII/II) = E1/2([CoIII/II(4′-
OMe-terpy)2]2+) − E1/2([CoIII/II(4′-SO2Me-terpy)2]2+) =
0.18 V).
Overall, these results reveal that, although the 4′-aryl-terpy
complexes span a smaller potential window than previously
established complexes, they can allow for further fine-tuning
redox potentials of both metal- and ligand-centered events
when coupled with ligand modifications introduced through-
out the terpy unit as well.
Hammett Parameter Correlation with Co Complexes.
To quantitatively assess the effect of 4′-aryl ring substituents to
fine-tune the redox potentials of bis-4′-aryl-terpy Co
complexes, we chose to explore the reduction potentials of
these complexes as a function of the electronic nature of these
substituents, which was accomplished using the Hammett free-
energy relationships (eqs 1 and 2).44
The second class of complexes undergo two or three
reversible redox events, followed by an irreversible reduction
that is accompanied by an increase in anodic current (see
Figure 3B, as well as Figures S5−S8; here, L = 3,5-Br or 3-Br,
3,4,5-F, 2-CF3, 4-CF3). Previous dropping mercury electrode
AC and DC polarographic studies of a series of [M(terpy)2]2+
complexes revealed that terpy dissociation and subsequent 1 or
2e− reductions accounted for their complex behavior under
strongly reducing conditions (i.e., irreversibility following two
or three ligand-centered reductions of the metal complexes).43
We hypothesize that a similar phenomenon is occurring in the
second class of compounds presented here (Figure 3B, and
Figures S5−S8) for which one of the ligands is liberated
following three reductions (1 × metal-centered + 2 × ligand
centered reductions) and, upon ligand dissociation, additional
current is generated by the reduction of free ligand under the
strongly reducing conditions. The irreversibility observed at
the first ligand-centered reduction for [Co(3,5-Br)2]2+ could
be likewise justified. However, coordinated ligand decom-
position via the formation of a bromine radical as an alternate
decomposition mechanism cannot be discounted. It is also
important to note that a combination of electrochemical
behaviors presented above, as well as nonobvious ones, may be
occurring simultaneously.
Previous studies of metal complexes featuring 4′-substituted
terpyridine ligands by Slattery and co-workers revealed a
ΔE1/2(CoIII/II) = 0.36 V range in potentials between the
strongly electron releasing ligand N,N-diethyl-6-(pyridin-2-yl)-
[2,2′-bipyridin]-4-amine, 4′-NEt2-terpy, and the strongly
electron-deficient 4-methanesulfonyl-6-(pyridin-2-yl)-2,2′-bi-
pyridine, 4′-SO2Me-terpy (Figure 4).18 Examination of the
electrochemical data for the series of Co complexes
synthesized herein reveals a smaller electrochemical range for
the CoIII/II redox couple (0.116 V), which is consistent with
the increased distance of the 4′-aryl substituents from the
metal center. To provide a qualitative comparison to Slattery’s
work, we can inspect the following pair of complexes: a 3,4,5-F
(Σσ = 0.74) substitution pattern is similar to a 4′-SO2Me group
in 4′-SO2Me-terpy (Σσ = 0.72), in terms of Σσ values,
providing the most electron-deficient ligands in both series,
while the corresponding 4-methoxy-6-(pyridin-2-yl)-2,2′-bi-
pyridine), 4′-OMe-terpy, supported complexes will serve as
the most electron-donating representative species (see Figure
4). Using these complexes as reference points, it is quite
E
1/2(L)=ρΣσ + E1/2(H)
(1)
(2)
Σσ = 2 × (σm,3 + σp + σm,5
)
E
1/2(L) is the reduction potential associated with a metal
complex coordinated to a particular ligand L, E1/2(H)
represents the reduction potential of the unsubstituted ligand,
and Σσ is the sum of Hammett parameters for a given ligand.
Σσ is the sum of meta Hammett parameters at the 3-position,
σm,3, the 5-position, σm,5, and that of the para Hammett
parameter, σp (when the functional groups are not hydrogen),
and these values are multiplied by 2 to account for two
coordinated 4′-aryl substituted ligands per metal center. Thus,
ρ, which is an indicator of a given redox reaction’s sensitivity to
substituent changes per coordinated ligand, can be determined
by measuring the slope in Hammett potential plots.30,45
Beginning our analysis with cobalt complexes displaying
reversible reduction couples (Class 1), we examined the
E
1/2(CoIII/II), E1/2(CoII/I), E1/2(Lπ0/−1), and E1/2(L−π 1/−2), with
respect to the Σσ of each [Co(L)2]2+ (Figure 5). To summarize
the data, ρ is (1) smaller in metal-centered reductions than
ligand-based reductions with the smallest for E1/2(CoIII/II) (ρ =
0.037 V/Σσ) and (2) similar for E1/2(L0π/−1) (ρ = 0.097 V/Σσ)
and E1/2(L−π 1/−2) (ρ = 0.10 V/Σσ).
Perhaps remarkably, the redox potential versus Σσ linear
trend holds for multisubstituted (3,4,5-OMe and 3,4,5-F)
compounds as well as combinations of meta- and para-
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reduction potentials. Specifically, we were unable to determine
a relationship between reduction potentials and Taft48 or
Charton49 parameters, which were developed to supplement
the nonexistent ortho-substituted Hammett parameters by
correlating the ortho-substituent van der Waals radii to
hydrolysis reaction rates. Qualitatively, however, E1/2 values
of 2-substituted complexes of 2-CF3 and 2-OMe roughly
resemble their 4′-substituted analogues (see Tables S1 and
Table S2 for E1/2 values). Incorporating additional methoxy
groups (e.g., 2-OMe vs 2,4-OMe vs 2,4,6-OMe) generally
lowers the E1/2 values of all complexes, with the exception of
E
1/2(CoIII/II), which is consistent with increasing the electron
density of the overall ligand and metal complex.
Hammett Correlation with Mn, Fe, Ni, and Zn
Complexes. The correlations identified for the newly
synthesized [Co(L)2]2+ complexes motivated us to examine
whether the trends extend to related first-row transition-metal
complexes. To this end, we explored the analogous Mn, Fe, Ni,
and Zn complexes featuring the following four ligands: 3,5-
CF3, 3-CF3, 3-OMe, and 4-OMe, which were chosen to span
an identical electronic range (Σσ = +1.72, +0.86, +0.23, and
−0.536, respectively) as that examined for the cobalt
complexes (with the exception of the synthetically inaccessible
[Mn(3,5-CF3)2]2+).
Figure 6 contains a plot of the metal and ligand-centered
reduction potential vs Σσ for each metal series, while Table 1
lists their linear fits and regressions, including those of cobalt.
To summarize, all M(III/II) couples display relatively similar, but
small ρ values, ranging from 0.037 V/Σσ to 0.055 V/Σσ, while
all reductions display more pronounced responses with slopes
ranging from 0.063 V/Σσ to 0.10 V/Σσ over all metal and
ligand couples. Generally, ρ increases with additional
reductions (i.e., all E1/2(L−π 1/−2) have larger slopes than
Figure 5. A plot of reduction potentials for each reversible metal- and
ligand-centered couple in [Co(L)2]2+ as a function of the complexes
Σσ. The solid lines are the linear regression fit and their corresponding
line equation is presented above each of the couples, where ρ is equal
to the slope and indicates the sensitivity of the couple to change in
functional groups of the 4′-aryl ring. The linear regression (r2) is
presented below each fit. The Σσ value for each ligand is indicated at
the top of the plot.
E
1/2(L0π/−1) values). All E1/2 values produced r2 values of
>0.93, with the exception of the MnIII/II couple and the cobalt
E
1/2(L−π 1/−2).
Metal- and ligand-based reduction ρ-values were reported in
̈
2+
Sjodin’s examination of [Mn(4′-R-terpy)2] and [Fe(4′-R-
terpy)2]2+ and are summarized in Table 2, alongside Slattery’s
results.18,19 Similar to the attenuation noted in cobalt
porphyrin complexes above, all [M(4′-Aryl-terpy)2]2+ ρ-values
are 60−80% lower than those with direct 4′-terpy function-
alization in [M(4′-R-terpy)2]2+ complexes. The most signifi-
cant attenuation of 80+% is observed in the E1/2(MIII/II).
Rapid Computational Model. Developing a rapid
computational model to predict the reduction potential of a
[M(L)2]2+ complex could be advantageous, especially in those
cases where experimentally tabulated Hammett parameters are
not available and synthesis may not be economically feasible.
substituted aryl rings (3,5-OMe, 3,5-F, 3,5-CF3; and 3,4,5-
OMe, 3,4,5-F). Furthermore, the trend extends to scaffolds
featuring both an electron-donating group (i.e., negative σ) and
an electron-withdrawing group (i.e., positive σ) (e.g., when L =
3-CF3,4-OMe). The results highlight the versatility of such a
tool to predict redox potentials of metal complexes of this type.
Direct functionalization of terpy in [Co(4′-R-terpy)2]2+
results in a E1/2(CoIII/II) ρ = 0.13 V (linear regression, r2 =
0.87; calculated from reported data), 70% higher than the
correlation identified for [Co(4′-aryl-terpy)2]2+ presented
here, which is consistent with attenuated electronic commu-
nication between the latter ligands and the Co center.18 Similar
findings were reported for Co porphyrin complexes, where
functionalization of the β-pyrrole moiety of tetraphenylpor-
phyrin, TPP, affords large ρ values for the porphyrin ring
oxidation (ρ = 0.15−0.19 V) and reduction (ρ = 0.29−0.320
V),46 whereas para-substitution of the phenyl groups in cobalt
complexes of meso-porphyrins results in E1/2(CoIII/II) and
̈
We considered the method of Sjodin and co-workers
correlating relative metal or nitrogen charges on iron and
manganese [M(4′-R-terpy)2]2+ complexes with Natural Bond
Order analysis. To minimize computational cost of modeling
full complexes any time such analysis is targeted, we sought a
closely related model based on substituted 4-phenyl-pyridines,
aryl-py, as simplified analogues.19 Calculations were performed
using ORCA at the B3LYP level of theory with the def2-SVP
basis set.50−53 Optimized structures resulted in a torsion
between the aryl ring and the pyridine ring, which is a common
feature that is observed in single-crystal XRD structures, and
were coplanar when reduced by 1e−. Inspired by studies of
substituted ferrocenyl-containing β-diketones showing a
correlation between calculated LUMO energies and metal
E
1/2(CoII/I) (in butyronitrile) ρ values of 0.034 and 0.041 V,
respectively (∼75+% lower correlation).47
Among the cobalt complexes examined, the species featuring
ligands with ortho-substituents displayed no obvious quantita-
tive correlations between the type (inductive or resonance
conjugated) or number of substituents and the various
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CONCLUSIONS
■
A series of 4′-aryl-substituted terpy ligands, L, were prepared in
order to determine the influence of such ligand modifications
on the redox properties of homoleptic [M(L)2]2+ complexes.
Electrochemical investigations of Co complexes unveiled a
redox span between 0.12 and 0.36 V for the various cobalt
redox couples upon introduction of various electron-with-
drawing or electron-donating substituents on the 4′-aryl group.
Good correlations between the Hammett parameters of the
substituted terpy ligands and the corresponding metal- and
ligand-based reduction potentials were identified and trans-
lated well to other [M(L)2]2+ (M = Mn, Fe, Ni, Zn)
complexes. The Hammett analysis revealed that the phenyl-
spacer results in 60−80% attenuation of electron density,
compared to their 4′-R-terpy analogue, which is dependent on
the metal and redox couple analyzed. Using 4-aryl-substituted
pyridines as models, computationally determined LUMO
energies and reduction potentials of these pyridine derivatives
correlated well with their Hammett parameters, offering a
simple predictive tool for assessing redox potentials of M(L)22+.
Overall, these studies highlight the potential possibility of using
4′-aryl substituents as an additional approach to fine-tune the
redox potential of bis-terpyridine metal complexes and could
offer new avenues in photochemical and electrochemical
catalytic applications. Future work will focus on further
expanding these relationships to heteroleptic (i.e., mixed-
terpy) metal complexes, as well as those of second-row
transition metals.
EXPERIMENTAL SECTION
■
Instrumentation. Electrochemical measurements were conducted
using a CH Instruments Model 760E potentiostat. UV/vis spectra
were recorded on an Agilent Technologies Model 8453 diode-array
spectrophotometer. NMR spectra were collected on a JEOL Model
JNM-ECZ400S-CON 400 MHz, 9.4 T spectrometer. All 1H and
13C{1H} chemical shifts were referenced to residual solvent signals:
7.26 and 77.16 ppm for CDCl3; 2.05 and 29.84 ppm for acetone-d6;
2.50 and 39.52 ppm for DMSO-d6, respectively.54 19F resonances were
Figure 6. Plot of reduction potentials for various [M(L)2]2+
complexes. Blue shapes indicate nickel complexes, red indicates
manganese complexes, purple indicates iron complexes, and green
indicate zinc complexes. Square symbols indicate E1/2(MIII/II), circular
symbols indicate E1/2(L0π/−1), and diamonds indicate E1/2(L−π 1/−2).
The linear regression (r2) values and fits are presented in Table 1. The
coordinated ligand and their Σσ are depicted above and below the
plot, respectively.
determined by using PF−6 as an internal standard (−68.3 ppm, JPF
=
711 Hz), which was determined by referencing to a known standard,
inserting a sealed melting point capillary tube filled with trifluoro-
acetic acid (−76.55 ppm) into an NMR tube. Ligand fluorine
resonances were also determined by inserting the capillary into the
NMR sample. FT-IR data was collected on a Shimadzu IR Affinity-1
with a Pike Technologies Multiple Reflection ZnSe Trough Plate
Horizontally Attenuated Total Reflectance (HATR) accessory.
HATR-IR samples were prepared by dissolving a small amount of
metal complexes (<10 mg) in a minimal amount of MeCN (∼1 mL).
The sample solution was then deposited dropwise along the entire
length of the HATR trough, using a Pasteur pipet, and the MeCN was
left to evaporate. Once the sample appeared dry, another layer was
added and the process repeated for a combined total of three layers. A
gentle stream of compressed air was run over the HATR to evaporate
the remaining MeCN. New ligands (L = 2,6-F and 3-CF3,4-OMe) IR
data were collected with a ThermoFisher Scientific Nicolet iS20 FTIR
spectrometer with a monolithic diamond ATR crystal mounted on the
Smart iTX ATR accessory. High-resolution mass spectrometry was
performed using an Agilent Model 6220 Accurate-Mass TOF HPLC/
MS with ESI probe (in positive mode) and an Agilent Model 1200
HPLC system using isocratic 70/30 MeOH/H2O + 0.1% formic acid
as solvent. Samples were injected directly into the MS using 1367B
Agilent Autosampler and matched the expected value within 5 ppm.
Note: [Mn(L)2][PF6]2 decomposed to free ligand under the acidic
HRMS conditions. Elemental analysis was performed using a Costech
Elemental Combustion System (ECS) 4010 and agree to within 0.4%
for C and N. Note: hydrogen percentages were not measurable with
redox potential, we sought to investigate whether the Hammett
relationships noted in the [Co(L)2]2+ complexes reflect the
energy of the LUMO of the system.28 Thus, we examined the
LUMO energies of aryl-py compounds (n = 22; 12 analogous
to the 4′-aryl-terpy and 10 with additional functionalities)
containing trifluoromethyl, methoxy, fluoro, bromo, amine,
hydroxy, thio, thiomethyl, methyl sulfoxide, methyl sulfone,
and methyl-substituents to their corresponding Σσ values (see
Table S13 in the Supporting Information for a complete list)
and found a strong correlation with the corresponding
Hammett values (r2 = 0.92; Figure 7). Furthermore, analysis
of the aryl-py E1/2(L0π/−1) vs Σσ for the 12 [M(L)2]2+ analogues
also shows a strong correlation (r2 = 0.92; see Table S14 in the
Supporting Information). The strong linear relationship
between the LUMO and E1/2(L0π/−1
)
vs Σσ indicates that
the simple aryl-py system can indeed serve as a rapid model to
predict the reduction potentials of 4′-aryl-terpy complexes,
when linked to the experimentally observed relationship
between [M(L)2]2+ redox potentials and Σσ. We are currently
investigating whether these trends hold with 4′-aryl-terpy
ligands themselves.
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Table 1. Table of ρ Values Associated with Each of the Metal and Ligand-Based Redox Potentials for All of the [M(L)2]2+
Complexes Investigated in This Study (Glassy Carbon Electrode)
couple
NiIII/II
MnIII/II
FeIII/II
CoIII/II
CoII/I
slope (in V/Σσ)
intercept (in V)
adj. r2
number of samples, n
a
0.0430 0.003
0.057 0.02
0.041 0.001
0.037 0.002
0.058 0.003
0.069 0.004
0.060 0.004
0.068 0.005
0.078 0.007
0.064 0.008
0.092 0.009
0.074 0.01
0.078 0.01
0.097 0.007
0.10 0.01
1.221 0.003
0.80 0.01
0.982
0.814
4
b
c
3
0.687 0.001
−0.148 0.002
−1.161 0.003
−1.501 0.002
−1.597 0.004
−1.621 0.005
−1.626 0.007
−1.732 0.008
−1.773 0.009
−1.780 0.007
−1.79 0.01
0.999
0.979
0.976
0.994
0.986
0.984
0.978
0.950
0.972
0.957
0.933
0.952
0.897
4
12
12
3
4
4
Mn(L0π/−1
Ni(L0π/−1)
)
)
Fe(L0π/−1
Zn(L0π/−1
Fe(L−π 1/−2
)
)
4
4
)
)
Zn(L−π 1/−2
Mn(L−π 1/−2
Ni(L−π 1/−2
Co(L0π/−1
Co(L−π 1/−2
4
3
c
)
)
4
d
)
−1.982 0.007
−2.31 0.01
11
e
)
7
a
b
[Ni(3,5-CF3)2][PF6]2 and [Ni(3-CF3)2][PF6]2 couples display peak to peaks >100 mV. All Mn(III/II) couples display peak to peaks >100 mV.
We were unable to synthesize [Mn(3,5-CF3)2][PF6]2 via the general procedure. The [Co(3,5-Br)2][PF6]2 couple is irreversible. [Co(3-
Br)2][PF6]2, [Co(4-CF3)2][PF6]2, [Co(3,4,5-F)2][PF6]2, and [Co(3,5-Br)2][PF6]2 couples are irreversible.
c
d
e
Table 2. Table of ρ Values Associated with Each of the Metal- and Ligand-Based Redox Potentials for Previously Reported
a
[M(4′-R-terpy)2]2+ Complexes of Mn, Fe, and Co
b
c
couple
MnIII/II
slope (in V/Σσ)
intercept (in V)
adj. r2
n
electrode
ref
0.29 0.03
0.24 0.02
0.17 0.03
0.21 0.03
0.25 0.02
0.17 0.01
0.17 0.03
0.13 0.02
0.89 0.03
−1.54 0.02
−1.83 0.03
0.96
0.96
0.87
0.88
0.97
0.99
0.89
0.87
5
GCE
GCE
GCE
Pt
GCE
GCE
GCE
Pt
19
19
19
18
19
19
19
18
Mn(L0π/−1
)
5
5
9
5
5
5
9
Mn(L−π 1/−2
FeIII/II
FeIII/II
)
d
1.05 0.03
0.69 0.02
−1.63 0.01
−1.78 0.03
Fe(L0π/−1
)
Fe(L−π 1/−2
)
d
CoIII/II
−0.27 0.02
a
All couples are reported versus [FeCp2]+/FeCp2 in 0.1 M TBAPF6 in MeCN unless otherwise noted. GCE = glassy carbon electrode, Pt =
b
c
d
platinum electrode. All Mn(III/II) couples display peak to peaks >100 mV; n = number of samples; vs SSCE.
Figure 7. Plot of LUMO energy (kcal) vs Σσ for 22 Ar-Py. Blue triangles are Ar-Py compounds with analogous [Co(L)2]2+ complexes synthesized.
Red diamonds are additional Ar-Py without analogously synthesized complexes. The green line is the regression line of all 22 Ar-Py.
the system’s current configuration. Single-crystal X-ray crystallography
was performed on a Rigaku Corporation XtaLAB mini (ROW) at 170
K with an Oxford Cryosystems Model Cryostream 800, accompanied
by Oxford AD61 Dry Air Unit, processed with CrysAlisPro, and
solved with ShelX using Olex2 1.3.55−57
Electrochemical Analysis. Electrochemical experiments were
performed in a three-compartment glass cell separated by medium- or
fine-porosity fritted funnels. Unless otherwise noted, a glassy carbon
electrode was suspended in the center compartment with a septum
containing 2 predrilled holes: one for the GC electrode and one for
the degassing tube. In one outer compartment, a reference electrode
was suspended in a septum containing 2 predrilled holes: one for the
reference electrode and one for a degassing tube. In the remaining
outer compartment, platinum wire or gauze was threaded through a
septum adjacent to a degassing tube and served as the counter
electrode. Prior to use, the cells were rinsed with MeCN and the
appropriate solutions were placed in their respective compartments.
The solutions were freed of oxygen by vigorously degassing the
solutions with N2 for a minimum of 5 min after the N2 flowed through
gas washers containing MeCN. The 3 mm diameter GC electrodes,
purchased from CH Instruments, were polished for ∼10 s with 0.3 μm
alumina polish on a polishing pad and rinsed with water, followed by
MeCN. Prior to performing electrochemical experiments, the Ag wire
reference electrode solution was replaced with a stock solution of 10
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mM AgNO3 in 0.1 M TBAH/MeCN. All [M(L)2]2+ compounds are
free of impurities via CV analysis.
collected via filtration over a 350-mL medium-porosity fritted glass
funnel and washed with EtOH (∼20−35 mL) until the EtOH filtrate
was clear. The process usually required 2 or 3 portions of ethanol and
would leave behind a pale yellow or white precipitate. Less was used
when the product was fairly soluble in EtOH. The precipitate was
then air-dried by continuing to pull vacuum on the filtration setup for
at least 30 min. The material was recrystallized with a minimal
amount of EtOH and the drying process was repeated when
Crystal Growth Details. All samples were grown via vapor−vapor
diffusion by dissolving 15−45 mg of a given complex in several
milliliters of acetonitrile in small test tubes resting in a capped bottle
containing diethyl ether to give the PF−6 salt. In some cases, the salt
was dissolved in acetone, which co-crystallized. [Co(2,4-OMe)2]2+
perchlorate exchange and subsequent crystallization was accomplished
by dissolving ∼15 mg of PF−6 salt in acetonitrile and layered on top of
∼2 mL of ∼1 M NaClO4. [Co(3,4,5-OMe)2]2+ and [Co(4-OMe)2]2+
were grown in a similar manner to confirm the structures previously
published.38,39 In many cases, the dissolving solvent co-crystallized
with the named complexes.
1
recrystallization was indicated by impurities in H NMR.
New Ligands. 3-CF3,4-OMe. 4-[4-methoxy-3-(trifluoromethyl)-
phenyl]-6-(pyridin-2-yl)-2,2′-bipyridine, 4-methoxy-3-trifluorome-
thylbenzaldehyde (5.00 g, 24.49 mmol), acetyl pyridine (6.57 g,
54.27 mmol, 2.21 equiv), EtOH (150 mL), NH4OH (150 mL), KOH
(3.08 g, 54.90 mmol, 2.24 equiv). Four day reflux. White powder
(4.99 g, 12.25 mmol, 50% yield). 1H (400 MHz, DMSO-d6) δ 8.80−
8.71 (m, 2H), 8.64 (m, 4H), 8.21 (dd, J = 8.7, 2.4 Hz, 1H, H6′′′),
8.08−7.97 (m, 3H), 7.52 (ddd, J = 7.6, 4.8, 1.2 Hz, 2H), 7.45 (d, J =
8.8 Hz, 1H, H5′′′), 3.99 (s, 3H, −OCH3). 13C{1H} (101 MHz,
DMSO-d6) δ 157.98 (C4′′′), 155.73, 154.90, 149.37, 147.82, 137.50,
132.93 (C6′′′), 129.52, 125.11, 125.06, 124.60, 120.99, 117.63 (q,
1JCF = 30.5 Hz, −CF3), 117.62, 113.88 (C5′′′), 56.51 (−OCH3). 19F
(376 MHz, DMSO-d6) δ −59.23. HR-MS (ESI+) m/z calc’d for
C23H17F3N3O 408.1318, found 408.1303 ([M + H]+). m.p.: 169−171
°C.
Computational Details. Calculations were performed in the
ORCA software suite (version 4.2.1) at the B3LYP level of theory
with the def2-SVP basis set scheme.50−53 Frequency calculations were
performed at the same level of theory to ensure all optimized
structures were minima. In several cases, we were unable to eradicate
the imaginary frequencies, despite exhaustive perturbation-then-
restarts, the introduction of dampening fields, grid scale adjustment,
etc. and thus treat those coordinates produced, and listed in the XYZ-
coordinate tables given in the Supporting Information, as approximate
minima for our analysis. Redox calculations were performed using the
standard Born−Haber cycle treatment according to the method
described by Castro and Bu
̈
hl.58 Optimized structures in Figure 7
2,6-F. 4-(2,6-difluorophenyl)-6-(pyridin-2-yl)-2,2′-bipyridine, 2,6-
difluorobenzaldehyde (3.29 g, 23.2 mmol), 2-acetyl pyridine (6.67 g,
59.5 mmol, 2.57 equiv), KOH (2.97 g, 52.9 mmol, 2.29 equiv), EtOH
(115 mL), NH4OH (100 mL). Four day reflux. Pale white powder
were produced with UCSF Chimera.59 MarvinSketch (MarvinSketch
19.22.0, ChemAxon (https://www.chemaxon.com) was used for
drawing, displaying, naming chemical structures, and NMR resonance
prediction.
1
(5.14 g, 14.88 mmol, 64% yield). H (400 MHz, CDCl3) δ 8.69
Materials. Anhydrous acetonitrile (ACROS Organics (Extra Dry,
Acroseal) or MilliporeSigma), other solvents, and chemicals were
used as received from Fisher Scientific. Tetra-n-butylammonium
hexafluorophosphate was recrystallized twice from ethanol (∼3 mL of
ethanol were used per gram of electrolyte) and the needles were
collected on a fritted funnel and air-dried by pulling a vacuum through
the collection for several hours.
(ddd, J = 4.8, 1.9, 0.9 Hz, 2H, H6), 8.66 (dt, J = 8.0, 1.1 Hz, 2H, H3),
8.57 (t, J = 1.3 Hz, 2H, H3′), 7.86 (td, J = 7.7, 1.8 Hz, 2H, H4),
7.44−7.27 (m, 3H, H4′′′ and H5), 7.09−6.95 (m, 2H, H3′′′).
4,1
13C{1H} (101 MHz, CDCl3) δ 160.29 (dd,
J
CF
= 250.6, 6.7 Hz,
C2′′′),156.06 (C2’), 155.71 (C2), 149.34 (C6), 139.73 (C4’), 137.03
4
(C4), 130.34 (t, JCF = 10.4 Hz, C4′′′), 124.04 (C5), 122.49 (C3′),
121.42 (C3), 116.67 (t, 3JCF = 18.2 Hz, C1′′′), 111.90 (d, 3JCF = 25.5
Hz, C3′′′). 19F (376 MHz, CDCl3) δ −112.12. HR-MS (ESI+) m/z
calc’d for C21H14F2N+3 346.1150, found 346.1134 ([M + H]+). m.p.:
199−201 °C.
Synthetic Details. The characterization of L = 2-OMe,60 3-
OMe,61 4-OMe,61 3,5-OMe,62 3,4,5-OMe,63 2,4-OMe,64 2,4,6-
OMe,63 3,4,5-F,65 2-CF3,66 3-CF3,67 4-CF3,68 3,5-F,69 3,5-CF3,69
3-Br,70 and 3,5-Br71 is consistent with that previously reported. To
the best of our knowledge, 2,6-F and 3-CF3,4-OMe have not been
Previously Reported Ligands. 2-CF3. 6-(pyridin-2-yl)-4-[2-
(trifluoromethyl)phenyl]-2,2′-bipyridine. 2-(trifluoromethyl)-benzal-
dehdye (5.58 g, 32.05 mmol), acetyl pyridine (7.8 g, 63.87
mmol,1.99 equiv), EtOH (128 mL), NH4OH (160 mL), KOH (3.7
g, 65.95 mmol, 2.06 equiv). Two day reflux. Pale yellow powder (5.31
g, 14.07 mmol, 44% yield).
3-CF3. 6-(pyridin-2-yl)-4-[3-(trifluoromethyl)phenyl]-2,2′-bipyri-
dine. 3-(trifluoromethyl)-benzaldehyde (4.37 g, 25.1 mmol), acetyl
pyridine (6.66 g, 55.0 mmol, 2.19 equiv), EtOH (80 mL), NH4OH
(100 mmol), KOH (3.14 g, 56.05 mmol, 2.24 equiv). Three day
reflux. White powder (4.43 g, 11.74 mmol, 47% yield).
4-CF3. 6-(pyridin-2-yl)-4-[4-(trifluoromethyl)phenyl]-2,2′-bipyri-
dine. 4-(trifluoromethyl)-benzaldehyde (4.385 g, 25.2 mmol), acetyl
pyridine (6.70 g, 55.3 mmol, 2.19 equiv), EtOH (80 mL), NH4OH
(100 mL), KOH (3.178 g, 56.7 mmol, 2.25 equiv). Six day reflux.
White powder (4.54 g, 12.03 mmol, 48% yield).
3,5-CF3. 4-[3,5-bis(trifluoromethyl)phenyl]-6-(pyridin-2-yl)-2,2′-
bipyridine. 3,5-bis(trifluoromethyl)benzaldehyde (5.26 g, 21.73
mmol), acetyl pyridine (6.28 g, 51.84 mmol, 2.38 equiv), EtOH
(80 mL), NH4OH (100 mL), KOH (2.67 g, 47.59 mmol, 2.19 equiv).
Eight day reflux. White powder (5.21 g, 11.70 mmol, 54% yield).
3,5-F. 4-(3,5-difluorophenyl)-6-(pyridin-2-yl)-2,2′-bipyridine. 3,5-
difluorobenzaldehyde (3.24 g, 22.8 mmol), acetyl pyridine (6.08 g,
50.02 mmol, 2.2 equiv), EtOH (80 mL), NH4OH (105 mL). (2.83 g,
50.51 mmol, 2.22 equiv) 4 day reflux. Fluffy white solid (5.23 g, 15.14
mmol, 66% yield).
1
reported previously. H, 13C{1H}, 19F, and 2D NMR (DQF COSY,
NOESY, TOCSY, HSQC, HSQC-TOCSY, and HMBC) spectra are
presented in the Supporting Information for these new ligands. In all
cases, terpyridines were prepared according to the General Ligand
Procedure below, with the exception of 2,4,6-OMe, which follows an
alternate procedure previously reported.63 With the exception of
[Co(3,4,5-OMe)2]2+, [Co(3-CF3)2]2+, [Co(4-OMe)2]2+, and [Co(2-
CF3)2]2+, [Co(3-CF3)2]2+, [Co(4-CF3)2]2+, all cobalt complexes
presented here, are new and were synthesized according to the general
metalation procedure described below.38,39 Complexes of Mn, Fe, Ni,
and Zn were also synthesized according to the general metalation
procedure. All [M(L)2]2+ complexes are new, with the exceptions of
[Ni(4-CF3)2]2+, [Ni(4-OMe)2]2+, [Mn(4-OMe)2]2+, [Zn(4-
OMe)2]2+, [Fe(4-OMe)2]2+, and [Fe(3-CF3)2]2+.33,36,38,39,67,72,73
General Ligand Synthetic Procedure. The substituted
benzaldehyde of interest was added to a 1 L round-bottom flask
and diluted with ethanol, EtOH (100 mL). Next, 2-acetyl pyridine
was added, followed by aqueous ammonium hydroxide (80 mL).
KOH was freshly ground to a fine powder with a mortar and pestle
and then added to the flask. A small amount of NH4OH (∼20 mL)
was used to rinse any KOH powder from the ground glass joint of the
1 L round-bottom flask, to prevent it from fusing to the condensing
column. Immediately after the addition and rapid dissolution of KOH,
the reaction solution turned a deep yellow. In most cases, a precipitate
formed within 15 min. When the precipitate was sufficiently thick to
prevent stirring, additional volumes of EtOH and NH4OH were
added to allow the solutions to flow freely with a 1-in. stirbar. The
reactions were then brought to reflux for 3−6 days, to accommodate
undergraduate schedules. The reaction precipitate would then be
3,4,5-F. 6-(pyridin-2-yl)-4-(3,4,5-trifluorophenyl)-2,2′-bipyridine.
3,4,5-trifluoro-benzaldehyde (4.02 g, 25.1 mmol), acetyl pyridine
(6.66 g, 55.0 mmol, 2.2 equiv), EtOH (80 mL). NH4OH (100 mL).
KOH (3.0851 g, 54.99 mmol, 2.19 equiv). Two day reflux. Pale yellow
solid (4.17 g, 11.48 mmol, 46% yield).
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2-OMe. 4-(2-methoxyphenyl)-6-(pyridin-2-yl)-2,2′-bipyridine. 2-
methoxybenzaldehyde (6.04 g, 44.37 mmol), acetyl pyridine (10.66
g, 88.0 mmol, 1.98 equiv), EtOH (160 mL), NH4OH (200 mL),
KOH (4.4566 g, 79.43 mmol, 1.79 equiv). Three day reflux. Oil
recovered, 155 mL EtOH, 6 mL conc. HCl added, white powder
precipitates out (2.75 g, 8.10 mmol, 18% yield).
3-OMe. 4-(3-methoxyphenyl)-6-(pyridin-2-yl)-2,2′-bipyridine. 3-
methoxybenxaldehyde (6.04 g, 44.00 mmol), acetyl pyridine (10.80
g, 89.00 mmol, 2.01 equiv), EtOH (160 mL), NH4OH (200 mL),
KOH (4.60 g, 82.00 mmol, 1.89 equiv). Three day reflux. Oil
recovered, EtOH added, conc. HCl added until ppt forms. White
powder (3.39 g, 10.00 mmol, 23% yield).
4-OMe. 4-(4-methoxyphenyl)-6-(pyridin-2-yl)-2,2′-bipyridine. 4-
methoxybenzaldehyde (6.28 g, 46.11 mmol), acetyl pyridine (9.83
g, 81.13 mmol, 1.76 equiv), EtOH (160 mL), NH4OH (200 mL),
KOH (4.56 g, 81.27 mmol, 1.76 equiv). Three day reflux. White solid
(6.62 g, 19.51 mmol, 42% yield).
3,5-OMe. 4-(3,5-dimethoxyphenyl)-6-(pyridin-2-yl)-2,2′-bipyri-
dine. 3,5-dimethoxy-benzaldehyde (4.17 g, 25.10 mmol), acetyl
pyridine (7.78 g, 64.19 mmol, 2.56 equiv), EtOH (80 mL),
NH4OH (100 mL), KOH (3.14 g, 55.93 mmol, 2.23 equiv). One
day reflux. White solid (4.64 g, 50% yield).
2,4-OMe. 4-(2,4-dimethoxyphenyl)-6-(pyridin-2-yl)-2,2′-bipyri-
dine. 2,4-dimethoxy-benzaldehyde (4.16 g, 25.02 mmol), acetyl
pyridine (7.78 g, 64.19 mmol, 2.57 equiv), EtOH (80 mL),
NH4OH (100 mL), KOH (3.09 g, 55.00 mmol, 2.20 equiv). Three
day reflux. White solid (3.53 g, 38% yield).
3,4,5-OMe. 6-(pyridin-2-yl)-4-(3,4,5-trimethoxyphenyl)-2,2′-bi-
pyridine. 3,4,5-trimethoxy-benzaldehyde (4.93 g, 25.15 mmol), acetyl
pyridine (7.78 g, 64.19 mmol, 2.57 equiv), EtOH (80 mL), NH4OH
(100 mL), KOH (3.12 g, 55.61 mmol, 2.11 equiv). Two day reflux.
White solid (3.03 g, 30% yield).
3-Br. 6-(pyridin-2-yl)-4-[3-bromophenyl]-2,2′-bipyridine. 3-bro-
mobenzaldehyde (3.69 g, 19.96 mmol), acetyl pyridine (6.26 g,
51.71 mmol, 2.6 equiv), EtOH (60 mL), NH4OH (75 mL), KOH
(2.51 g, 44.73 mmol, 2.2 equiv). Three day reflux. White solid (4.19 g,
54% yield).
3,5-Br. 4-(3,5-dibromophenyl)-6-(pyridin-2-yl)-2,2′-bipyridine.
3,5-dibromobenzaldehyde (5.00 g, 18.95 mmol), acetyl pyridine
(5.99 g, 49.41 mmol, 2.6 equiv), EtOH (80 mL), NH4OH (100 mL),
KOH (2.40g, 42.72 mmol, 2.3 equiv). Six day reflux. White solid
(4.72 g, 53% yield).
by diethyl ether two times. The solid was collected and
dissolved in minimal MeCN and precipitated by the slow
addition of diethyl ether. The resulting solid was filtered and
washed twice with diethyl ether.
New Complexes. [Co(3-CF3,4-OMe)2][PF6]2. 3-CF3,4-
OMe (0.203 g, 0.50 mmol), CoCl2·6H2O (0.053 g, 0.22 mmol,
0.45 equiv), MeOH (40 mL), H2O (20 mL). 2.5 h reflux.
Orange solid (0.241 g, 0.207 mmol, 94% yield). 1H NMR (400
MHz, acetone-d6) δ 92.93, 54.72, 42.83, 32.55, 32.55, 14.15 (d,
J = 7.6 Hz), 14.11, 9.52, 9.09, 5.05. 19F NMR (376 MHz,
acetone-d6) δ −57.32 (s, 12F, −CF3), −68.30 (d, J = 711 Hz,
12F, PF−6 ). In situ-[Co(3-CF3,4-OMe)2][PF6]2[BF4] 1H NMR
(400 MHz, acetone-d6) δ 9.72 (s, 2H, H3′), 9.09 (dd, J = 8.0,
1.5 Hz, 2H), 8.74 (dd, J = 8.8, 2.5 Hz, 1H), 8.63 (d, J = 2.37
(td, J = 7.8, 1.3 Hz, 2H), 7.95 (dd, J = 5.8, 1.3 Hz, 2H), 7.68
(d, J = 8.8 Hz, 1H), 7.56 (ddd, J = 7.5, 5.8, 1.4 Hz, 2H), 4.16
(s, 3H, −OCH3). HR-MS (ESI+) m/z calc’d for
C46H32CoF6N6O22+ 436.5906, found 436.5898 ([M -
2PF6]2+). Anal. Calc’d for C46H32CoF18N6O2P2: C, 47.48; N,
7.22. Found: C, 47.55; N, 7.01.
[Co(3,5-CF3)2][PF6]2. 3,5-CF3 (0.203 g, 0.455 mmol),
CoCl2 · 6H2O (0.050 g, 0.211 mmol, 0.463 equiv), MeOH (40
mL), H2O (20 mL). 4.5 h reflux. Orange solid (0.209 g, 0.169
1
mmol, 80% yield). H NMR (400 MHz, DMSO-d6) δ 82.00,
46.95, 30.69, 28.70, 13.33, 11.00, 10.73. 19F NMR (376 MHz,
DMSO-d6) δ −57.68 (s, 12F, −CF3), −68.30 (d, J = 711 Hz,
12F, PF−6 ). In situ-[Co(3,5-CF3)2][PF6]2[BF4] 1H NMR (400
MHz, DMSO-d6) δ 9.88 (s, 2H, H3′), 9.14 (d, J = 7.9 Hz,
2H), 9.08 (s, 2H, H2′′′), 8.59 (s, 1H, H4′′′), 8.40 (t, J = 7.8
Hz, 2H), 7.61 (d, J = 5.8 Hz, 2H), 7.50 (t, J = 6.9 Hz, 2H).
HR-MS (ESI+) m/z calc’d for C46H26CoF12N26+ 474.5674,
found 474.5683 ([M − 2PF6]2+). Anal. Calc’d for
C46H32CoF18N6O2P2: C, 44.57; N, 6.78. Found: C, 44.40; N,
6.51.
[Co(2,6-F)2][PF6]2. 2,6-F (0.205 g, 0.594 mmol), CoCl2·
6H2O (0.067 g, 0.282 mmol, 0.47 equiv), MeOH (50 mL),
H2O (25 mL). Three hour reflux. Orange solid (0.196 g, 0.189
mmol, 67% yield). 1H NMR (400 MHz, DMSO-d6) δ 97.61,
56.86, 45.67, 34.09, 9.92, 9.42, 8.93. 19F NMR (376 MHz,
DMSO-d6) δ −68.30 (d, J = 711 Hz, 12F, PF−6 ), −105.74 (s,
2,4,6-OMe. 6-(pyridin-2-yl)-4-(2,4,6-trimethoxyphenyl)-2,2′-bi-
pyridine. An alternative synthetic procedure was required for the
title compound and is as follows: NaOH (1.637 g, 40.92 mmol) was
added to PEG-300 (30 mL) in a 100 mL round-bottom flask at 0 °C
for 10 min. 2-Acetyl pyridine (5.94 g, 48.93 mmol) was added and
stirred at 0 °C and allowed to stir for 40 min at which point 2,4,6-
(OMe)3-benzaldehyde (4.056 g, 20.67 mmol) was added. After an
additional 3 h, the reaction was removed from the ice bath and
NH4OH (25 mL) was added to the reaction solution to turn green
and precipitate some material. The mixture was heated to reflux for 12
h and the white solid was then filtered using a 100-mL medium-
porosity fritted glass funnel and rinsed with ethanol until the filtrate is
clear resulting in a white powder, which was then recrystallized from
ethanol (2.43 g, 29% yield).
1
4F). in situ-[Co(2,6-F)2][PF6]2[BF4] H NMR (400 MHz,
DMSO-d6) δ 9.59 (s, 2H, H3′), 9.00 (d, J = 7.9 Hz, 2H), 8.31
(td, J = 7.7, 1.6 Hz, 2H), 7.84 (tt, J = 8.5, 6.4 Hz, 1H, H4′′′,
7.70−7.45 (m, 6H). HR-MS (ESI+) m/z calc’d for
C42H26CoF4N62+ 374.5738, found 374.5728 ([M − 2PF6]2+).
[Co(3,5-F)2][PF6]2. 3,5-F (0.195 g, 0.565 mmol), CoCl2·
6H2O (0.068 g, 0.286 mmol, 0.51 equiv), MeOH (20 mL),
H2O (10 mL). Three hour reflux. Orange solid (0.241 g, 0.232
mmol, 79% yield). 1H NMR (400 MHz, DMSO-d6) δ 83.93,
48.63, 31.62, 30.95, 12.92, 10.48, 10.29. 19F NMR (376 MHz,
DMSO-d6) δ −68.30 (d, J = 711 Hz, 12F, PF−6 ), −104.01 (s,
1
4F). In situ-[Co(3,5-F)2][PF6]2[BF4] H NMR (400 MHz,
METALATION PROCEDURE
■
DMSO-d6) δ 9.79 (s, 2H, H3′), 9.11 (d, J = 7.9 Hz, 2H), 8.40
(t, J = 7.8 Hz, 2H), 8.37−8.30 (m, 2H), 7.76 (tt, J = 9.1, 2.4
Hz, 1H, H4′′′), 7.60 (d, J = 5.7 Hz, 2H), 7.50 (t, J = 6.8 Hz,
2H). HR-MS (ESI+) m/z calc’d for C42H26CoF4N26+ 374.5738,
found 374.5748 ([M − 2PF6]2+).
General Complex Synthetic Procedure. Methanol was
added to a round-bottom flask containing the designated
ligand and degassed for 5 min with nitrogen. The precursor
metal salt was added to the methanol solution followed
immediately by water. The mixture was then refluxed for the
specified time and then was allowed to cool to room
temperature. A saturated solution of NH4PF6 in methanol
was added to induced precipitation of the orange-red title
compound. The solid was filtered using a medium porosity
fritted glass funnel and washed with deionized water followed
[Co(3,4,5-F)2][PF6]2. 3,4,5-F (0.205 g, 0.564 mmol),
CoCl2·6H2O (0.065 g, 0.273 mmol, 0.48 equiv), MeOH (20
mL), H2O (10 mL). Three hour reflux. Orange solid (0.256 g,
1
0.238 mmol, 87% yield). H NMR (400 MHz, DMSO-d6) δ
84.90, 49.19, 32.51, 31.14, 13.37, 10.44. 19F NMR (376 MHz,
DMSO-d6) δ −68.30 (d, J = 711 Hz, 12F, PF−6 ), −129.18 (s,
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1
4F), −157.28 (s, 2F). in situ-[Co(3,4,5-F)2][PF6]2[BF4] H
NMR (400 MHz, DMSO-d6) δ 8.92 (s, 2H, H3′), 8.21 (d, J =
8.0 Hz, 2H), 7.72 (dd, J = 9.1, 6.5 Hz, 2H), 7.55 (td, J = 7.8,
1.4 Hz, 2H), 6.71 (d, J = 5.8 Hz, 2H), 6.62 (ddd, J = 7.4, 5.8,
1.5 Hz, 2H). HR-MS (ESI+) m/z calc’d for C42H24CoF6N62+
392.5644, found 392.5652 ([M − 2PF6]2+). Anal. Calc’d for
C42H24CoF18N6P2: C, 46.90; N, 7.81. Found: C, 46.97; N,
7.57.
mL), H2O (25 mL). Refluxed 15 h. Orange solid (0.271 g,
0.249 mmol, 75% yield). H NMR (400 MHz, DMSO-d6) δ
1
84.70, 49.36, 33.17, 30.88, 12.29, 10.34, 9.32, 5.00. 19F NMR
(376 MHz, DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ). In
situ-[Co(3,5-OMe)2][PF6]2[BF4] 1H NMR (400 MHz,
DMSO-d6) δ 8.64 (s, 2H, H3′), 8.14 (d, J = 8.1 Hz, 2H),
7.34 (td, J = 7.8, 1.4 Hz, 2H), 6.65−6.55 (m, 4H), 6.47 (ddd, J
= 7.4, 5.8, 1.5 Hz, 2H), 5.92 (t, J = 2.2 Hz, 1H, H4′′′), 2.98 (s,
6H, −OCH3). HR-MS (ESI+) m/z calc’d for C46H38CoN6O42+
399.1153, found 399.1162 ([M − 2PF6]2+). Anal. Calc’d for
C46H38CoF12N6O4P2: C, 50.80; N, 7.73. Found: C, 50.68; N,
7.43.
[Co(2-OMe)2][PF6]2. 2-OMe (0.101 g, 0.297 mmol),
CoCl2·6H2O (0.0318 g, 0.134 mmol, 0.45 equiv), MeOH
(20 mL), H2O (10 mL). 2.5 h reflux. Orange solid (0.0936 g,
1
0.091 mmol, 68% yield). H NMR (400 MHz, DMSO-d6) δ
91.67, 54.21, 42.58, 32.35, 13.99, 10.04 (t, J = 7.9 Hz), 9.57,
8.96 (s, 1H), 8.84 (d, J = 8.3 Hz), 5.59. 19F NMR (376 MHz,
DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ). In situ-[Co(2-
OMe)2][PF6]2[BF4] 1H NMR (400 MHz, DMSO-d6) δ 9.58
(s, 2H, H3′), 9.08 (d, J = 8.3 Hz, 2H), 8.35 (td, J = 7.6, 1.7 Hz,
2H), 8.07 (dd, J = 7.7, 1.8 Hz, 1H), 7.75 (ddd, J = 8.7, 7.4, 1.7
Hz, 1H), 7.60−7.50 (m, 4H), 7.47 (d, J = 8.4 Hz, 1H), 7.39 (t,
J = 7.5 Hz, 1H), 4.07 (s, 3H, −OCH3). HR-MS (ESI+) m/z
calc’d for C44H34CoN6O22+ 368.6032, found 368.6039 ([M −
2PF6]2+).
[Co(3,4,5-OMe)2][PF6]2. 3,4,5-OMe (0.534 g, 1.33
mmol), CoCl2·6H2O (0.139 g, 0.586 mmol, 0.438 equiv),
MeOH (50 mL), H2O (25 mL). Five hour reflux. Orange solid
1
(0.664 g, 0.579 mmol, 96% yield). H NMR (400 MHz,
DMSO-d6) δ 85.23, 49.88, 34.56, 30.99, 12.53, 10.22, 5.33,
4.70. 19F NMR (376 MHz, DMSO-d6) δ −68.30 (d, J = 711
Hz, PF−6 ). In situ-[Co(3,4,5-OMe)2][PF6]2[BF4] 1H NMR
(400 MHz, DMSO-d6) δ 9.59 (s, 2H, H3′), 9.13 (dd, J = 7.9,
1.5 Hz, 2H), 8.38 (td, J = 7.8, 1.4 Hz, 2H), 7.70 (s, 2H, H2′′′),
7.61 (dd, J = 5.9, 1.3 Hz, 2H), 7.51 (ddd, J = 7.3, 5.8, 1.5 Hz,
2H), 4.09 (s, 6H, −OCH3), 3.87 (s, 3H, −OCH3). HR-MS
(ESI+) m/z calc’d for C48H42CoN6O26+ 428.6243, found
[Co(2,4-OMe)2][PF6]2. 2,4-OMe (0.253 g, 0.68 mmol),
CoCl2·6H2O (0.074 g, 0.31 mmol, 0.45 equiv), MeOH (50
mL), H2O (25 mL). Five hour reflux. Orange solid (0.294 g,
428.6246 ([M
C48H42CoF12N6O6P2: C, 50.23; N, 7.32. Found: C, 50.41; N,
7.18.
−
2PF6 ]2 + ). Anal. Calc’d for
1
0.27 mmol, 87% yield). H NMR (400 MHz, DMSO-d6) δ
89.95, 53.59, 41.89, 31.77, 14.09, 14.07, 9.57, 8.60, 8.58, 8.41,
8.40, 5.69, 4.96. 19F NMR (376 MHz, DMSO-d6) δ −68.30 (d,
[Co(3-Br)2][PF6]2. 3-Br (0.205 g, 0.528 mmol), CoCl2·
6H2O (0.061 g, 0.256 mmol, 0.49 equiv), MeOH (20 mL),
H2O (10 mL). Three hour reflux. Orange solid (0.240 g, 0.195
mmol, 89% yield). 1H NMR (400 MHz, DMSO-d6) δ 83.74,
1
J = 711 Hz, PF−6 ). In situ-[Co(2,4-OMe)2][PF6]2[BF4] H
NMR (400 MHz, DMSO-d6) δ 9.48 (s, 2H, H3′), 9.01 (d, J =
7.9 Hz, 2H), 8.30 (td, J = 7.7, 1.5 Hz, 2H), 8.10 (d, J = 8.5 Hz,
1H), 7.55 (dd, J = 5.8, 1.4 Hz, 2H), 7.49 (ddd, J = 7.4, 5.7, 1.5
Hz, 2H), 6.96 (overlapping dd+s, 2H), 4.07 (s, 3H), 3.97 (s,
3H, -OCH3). HR-MS (ESI+) m/z calc’d for C46H38CoN6O42+
398.6138, found 398.6142 ([M - 2PF6]2+).
48.66, 31.98, 30.81, 13.45, 13.13, 10.56, 10.54, 10.45, 8.59. 19
F
NMR (376 MHz, DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ).
1
in situ-[Co(3-Br)2][PF6]2[BF4] H NMR (400 MHz, DMSO-
d6) δ 9.80 (s, 2H, H3′), 9.17 (dd, J = 8.0, 1.5 Hz, 2H), 8.75
(t, J = 1.9 Hz, 1H, H2”’), 8.53 (d, J = 7.8 Hz, 1H), 8.40 (td, J =
7.8, 1.4 Hz, 2H), 8.01 (dd, J = 8.2, 1.9 Hz, 1H), 7.84 (t, J = 7.9
Hz, 1H, H5′′′), 7.61 (dd, J = 5.8, 1.3 Hz, 2H), 7.50 (ddd, J =
7.5, 5.8, 1.5 Hz, 2H). HR-MS (ESI+) m/z calc’d for
C42H28Br2CoN62+ 417.5024, found 417.5030 ([M − 2PF6]2+).
Anal. Calc’d for C42H28Br2CoF12N6P2: C, 44.83; N, 7.47.
Found: C, 44.58; N, 7.46.
[Co(2,4,6-OMe)2][PF6]2. 2,4,6-OMe (0.501 g, 1.26
mmol), CoCl2·δ6H2O (0.133 g, 0.561 mmol, 0.48 equiv),
MeOH (100 mL), H2O (50 mL). Four hour reflux. Orange
1
solid (0.500 g, 0.436 mmol, 78% yield). H NMR (400 MHz,
DMSO-d6) δ 99.84, 59.96, 54.07, 33.96, 8.87, 8.55, 6.49, 5.37.
19F NMR (376 MHz, DMSO-d6) δ −68.30 (d, J = 711 Hz,
1
PF−6 ). In situ-[Co(2,4,6-OMe)2][PF6]2[BF4] H NMR (400
MHz, DMSO-d6) δ 9.32 (s, 2H, H3′), 8.95 (dd, J = 8.0, 1.5
Hz, 2H), 8.28 (td, J = 7.8, 1.3 Hz, 2H), 7.55 (ddd, J = 7.5, 5.7,
1.5 Hz, 2H), 7.40 (d, J = 5.7 Hz, 2H), 6.58 (s, 2H, H3′′′/
H5′′′), 3.96 (9H, −OCH3). HR-MS (ESI+) m/z calc’d for
C48H42CoN6O62+ 428.6243, found 428.6251 ([M - 2PF6]2+).
[Co(3-OMe)2][PF6]2. 3-OMe (0.101 g, 0.296 mmol),
CoCl2·6H2O (0.032 g, 0.135 mmol, 0.46 equiv), MeOH (20
mL), H2O (10 mL). Three hour reflux. Orange solid (0.096 g,
[Co(3,5-Br)2][PF6]2. 3,5-Br (0.100 g, 0.215 mmol), CoCl2·
6H2O (0.032 g, 0135 mmol, 0.63 equiv), MeOH (25 mL),
H2O (12.5 mL). Three hour reflux. Orange solid (0.103 g,
1
0.080 mmol, 60% yield). H NMR (400 MHz, DMSO-d6) δ
82.94, 47.72, 30.74, 30.10, 13.23, 10.82, 10.58. 19F NMR (376
MHz, DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ). in situ-
[Co(3,5-Br)2][PF6]2[BF4] 1H NMR (400 MHz, DMSO-d6) δ
9.80 (s, 2H, H3′), 9.14 (d, J = 7.9 Hz, 2H), 8.75 (d, J = 1.7 Hz,
2H, H2”’), 8.40 (td, J = 7.8, 1.3 Hz, 2H), 8.32 (d, J = 1.8 Hz,
1H, H4”’), 7.58 (d, J = 6.0 Hz, 2H), 7.50 (ddd, J = 7.4, 5.8, 1.5
Hz, 2H). HR-MS (ESI+) m/z calc’d for C42H26Br4CoN62+
496.4119, found 496.4127 ([M - 2PF6]2+). Anal. Calc’d for
C42H26Br4CoF12N6P2: C, 39.31; N, 6.55. Found: C, 38.93; N,
6.31.
1
0.093 mmol, 69% yield). H NMR (400 MHz, DMSO-d6) δ
84.67, 49.51, 33.49, 30.88, 12.86, 12.81, 10.33, 9.88, 8.66, 5.03.
19F NMR (376 MHz, DMSO-d6) δ −68.30 (d, J = 711 Hz,
PF−6 ). In situ-[Co(3-OMe)2][PF6]2[BF4] 1H NMR (400 MHz,
DMSO-d6) δ 9.75 (s, 2H, H3′), 9.19 (d, J = 8.0 Hz, 2H), 8.38
(td, J = 7.8, 1.3 Hz, 2H), 8.11 (d, J = 7.6 Hz, 1H), 8.06 (t, J =
2.2 Hz, 1H, H2′′′), 7.79 (t, J = 8.0 Hz, 1H, H5′′′), 7.62 (d, J =
6.0 Hz, 2H), 7.50 (ddd, J = 7.5, 5.8, 1.5 Hz, 2H), 7.39 (dd, J =
8.4, 2.5 Hz, 1H), 4.02 (s, 3H, −OCH3). HR-MS (ESI+) m/z
calc’d for C44H34CoN6O22+ 368.6032, found 368.6039 ([M -
2PF6]2+).
[Mn(3-CF3)2][PF6]2. 3-CF3 (0.10 g, 0.26 mmol), MnCl2·
4H2O (0.024 g, 0.12 mmol, 0.45 equiv), MeOH (20 mL), H2O
(10 mL). Off-white solid (0.110 g, 0.100 mmol, 83%). 19F
NMR (376 MHz, DMSO-d6) δ −59.32 (s, 6F, −CF3),
−68.30 (d, J = 711 Hz, PF−6 ). HR-MS (ESI+) m/z calc’d for
C22H15F3N3 378.1213, found 378.1221 ([M − Mn − 2PF6 +
[Co(3,5-OMe)2][PF6]2. 3,5-OMe (0.253 g, 0.684 mmol),
CoCl2·6H2O (0.079 g, 0.33 mmol, 0.48 equiv), MeOH (50
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H]+). Anal. Calc’d for C44H28F18MnN6P2 · H2O: C, 47.29; N,
7.52. Found: C, 47.04; N, 7.74.
−59.05 (s, 12F, −CF3), −68.30 (d, J = 711 Hz, 12F, PF−6 ).
13C{1H} NMR (101 MHz, DMSO-d6) δ 152.18, 149.85,
148.02, 147.62, 141.54, 138.51, 131.59 (q, J = 33.0 Hz, −CF3),
129.35, 128.04, 127.54, 124.82, 124.59, 123.96, 122.32, 122.10,
119.39. HR-MS (ESI+) m/z calc’d for C46H26F12N6Zn2+
477.0654, found 477.0659 ([M − 2PF6]2+). Anal. Calc’d for
C46H26F24N6P2Zn: C, 44.34; N, 6.74. Found: C, 44.16; N,
6.51.
[Mn(3-OMe)2][PF6]2. 3-OMe (0.102 g, 0.301 mmol),
MnCl2·4H2O (0.0265, 0.121 mmol), MeOH (20 mL), H2O
(10 mL). Three hour reflux. (0.0707 g, 0.069 mmol, 46%
yield). 19F NMR (376 MHz, DMSO-d6) δ −68.30 (d, J = 711
Hz, PF6−). HR-MS (ESI+) m/z calc’d for C22H18N3O
340.1444, found 340.1452 ([M - Mn - 2PF6 + H]+). Anal.
Calc’d for C44H34F12MnN6O2P2: C, 51.63; N, 8.21. Found: C,
51.49; N, 7.97.
[Fe(3,5-CF3)2][PF6]2. 3,5-CF3 (0.11 g, 0.24 mmol),
(NH4)2Fe(SO4)2·6H2O (0.042 g, 0.11 mmol, 0.45 equiv),
MeOH (20 mL), H2O (10 mL). Four hour reflux. Purple solid
(0.11 g, 0.089 mmol, 81% yield). 1H NMR (400 MHz,
DMSO-d6) δ 9.82 (s, 2H, H3′), 9.19 (s, 2H, H2′′′), 9.09 (d, J
= 8.0 Hz, 2H), 8.51 (s, 1H, H4′′′), 8.07 (t, J = 7.8 Hz, 2H),
7.31 (d, J = 5.5 Hz, 2H), 7.21 (t, J = 6.6 Hz, 2H). 19F NMR
(376 MHz, DMSO-d6) δ −59.08 (s, 12F, −CF3), −68.30 (d, J
= 711 Hz, 12F, PF6−). 13C{1H} NMR (101 MHz, DMSO-d6) δ
160.28, 157.82, 152.95, 146.07, 139.05, 131.71 (q, J = 33.7 Hz,
−CF3), 128.94, 127.89, 124.94, 124.57, 124.06, 122.23, 121.97,
119.51. HR-MS (ESI+) m/z calc’d for C46H26F12FeN62+
473.0683, found 473.0692 ([M - 2PF6]2+).
[Zn(3-CF3)2][PF6]2. 3-CF3 (0.50 g, 1.3 mmol), ZnCl2
(0.086 g, 0.63 mmol, 0.48 equiv), MeOH (100 mL), H2O
(50 mL). Three hour reflux. Yellow solid (0.37 g, 0.333 mmol,
1
53% yield). H NMR (400 MHz, DMSO-d6) δ 9.46 (s, 2H,
H3′), 9.15 (d, J = 8.1 Hz, 2H), 8.74 (s, 1H), 8.69 (d, J = 7.9
Hz, 1H), 8.30 (t, J = 7.9 Hz, 2H), 8.10 (d, J = 7.9 Hz, 1H),
8.02 (t, J = 7.8 Hz, 1H), 7.97 (d, J = 5.1 Hz, 2H), 7.51 (dd, J =
7.6, 5.1 Hz, 2H). 19F NMR (376 MHz, DMSO-d6) δ −58.94
(−CF3, s, 6F), −68.30 (d, J = 711 Hz, 12F, PF−6 ). 13C{1H}
NMR (101 MHz, DMSO-d6) δ 153.59, 149.67, 147.94,
147.63, 141.40, 136.78, 132.35, 130.65, 130.31 (q, J = 31.6 Hz,
−CF3), 127.83, 125.55, 125.01, 123.74, 122.84, 121.72. HR-
MS (ESI+) m/z calc’d for C44H28F6N6Zn2+ 409.0780, found
409.0793 ([M − 2PF6]2+).
[Zn(3-OMe)2][PF6]2. 3-OMe (0.11 g, 0.31 mmol), ZnCl2
(0.019 g, 0.14 mmol, 0.46 equiv), MeOH (20 mL), H2O (10
mL). Three hour reflux. White solid (0.066 g, 0.064 mmol,
[Fe(3-OMe)2][PF6]2. 3-OMe (0.10 g, 0.30 mmol),
(NH4)2Fe(SO4)2·6H2O (0.052 g, 0.13 mmol, 0.45 equiv),
MeOH (20 mL), H2O (10 mL). Ten hour reflux. Purple solid
1
46% yield). H NMR (400 MHz, DMSO-d6) δ 9.34 (s, 2H,
1
(0.085 g, 0.083 mmol, 64% yield). H NMR (400 MHz,
H3′), 9.13 (s, 2H), 8.27 (t, J = 7.9 Hz, 2H), 7.96 (m, 4H), 7.69
(t, J = 8.0 Hz, 1H), 7.50 (t, J = 6.0 Hz, 2H), 7.30 (d, J = 8.6
Hz, 1H), 3.99 (s, 3H, −OCH3). 19F NMR (376 MHz, DMSO-
d6) δ −68.30 (d, J = 711 Hz, PF−6 ).13C{1H} NMR (101 MHz,
DMSO-d6) δ 160.25, 155.20, 149.57, 147.97, 147.81, 141.49,
137.16, 130.83, 127.86, 123.73, 121.46, 120.63, 116.50, 114.11,
55.72 (−OCH3). HR-MS (ESI+) m/z calc’d for
C44H34N6O2Zn2+ 371.1012, found 371.1021 ([M − 2PF6]2+).
Anal. Calc’d for C44H34F12N6O2P2Zn: C, 51.11; N, 8.13.
Found: C, 50.72; N, 7.85.
Previously Reported Complexes. [Co(2-CF3)2][PF6]2.
2-CF3 (0.101 g, 0.266 mmol), CoCl2·6H2O (0.0287 g, 0.12
mmol, 0.45 equiv), MeOH (20 mL), H2O (10 mL). One hour
reflux. Orange solid (0.11 g, 0.10 mmol, 83% yield).
[Co(3-CF3)2][PF6]2. 3-CF3 (0.109 g, 0.289 mmol), CoCl2·
6H2O (0.03 g, 0.126 mmol, 0.44 equiv), MeOH (20 mL), H2O
(10 mL). Five hour reflux. Orange solid (0.122 g, 0.11 mmol,
88% yield).
[Co(4-CF3)2][PF6]2. 4-CF3 (0.100 g, 0.266 mmol), CoCl2·
6H2O (0.0286 g, 0.120 mmol, 0.45 equiv), MeOH (20 mL),
H2O (10 mL). One hour reflux. Orange solid (0.083 g, 0.075
mmol, 63% yield).
[Co(4-OMe)2][PF6]2. 4-OMe (0.101 g, 0.297 mmol),
CoCl2·6H2O (0.032 g, 0.135 mmol, 0.45 equiv), MeOH (20
mL), H2O (10 mL). Three hour reflux. Orange solid (0.124 g,
0.121 mmol, 89% yield).
[Mn(4-OMe)2][PF6]2. 4-OMe (0.200 g, 0.589 mmol),
MnCl2·4H2O (0.053 g, 0.268 mmol), MeOH (20 mL), H2O
(10 mL). 3.5 h reflux. (0.230 g, 0.225 mmol, 76% yield).
[Fe(3-CF3)2][PF6]2. 3-CF3 (0.10 g, 0.27 mmol),
(NH4)2Fe(SO4)2·6H2O (0.050 g, 0.13 mmol, 0.48 equiv),
MeOH (20 mL), H2O (10 mL). One hour reflux. Purple solid
(0.043 g, 0.039 mmol, 30% yield).
[Fe(4-OMe)2][PF6]2. 4-OMe (0.10 g, 0.22 mmol),
(NH4)2Fe(SO4)2·6H2O (0.060 g, 0.15 mmol, 0.69 equiv),
MeOH (20 mL), H2O (10 mL). Three hour reflux. Purple
solid (0.103 g, 0.101 mmol, 67% yield).
DMSO-d6) δ 9.60 (s, 2H, H3′), 9.03 (d, J = 8.0 Hz, 2H),
8.14−7.92 (m, 4H), 7.71 (t, J = 7.9 Hz, 1H), 7.25 (m, 3H),
7.16 (t, J = 6.6 Hz, 2H), 4.00 (s, 3H, -OCH3). 19F NMR (376
MHz, DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ). 13C{1H}
NMR (101 MHz, DMSO-d6) δ 160.75, 160.41, 158.40,
153.33, 149.55, 139.31, 137.98, 131.24, 128.15, 124.70, 121.82,
120.74, 116.48, 114.19, 56.15 (−OCH3). 19F NMR (376 MHz,
DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ). HR-MS (ESI+)
m/z calc’d for C44H34FeN6O22+ 367.1041, found 367.1050 ([M
- 2PF6]2+). Anal. Calc’d for C44H34F12FeN6O2P2: C, 51.58; N,
8.20. Found: C, 51.34; N, 7.93.
[Ni(3,5-CF3)2][PF6]2. 3,5-CF3 (0.10 g, 0.22 mmol), NiCl2·
6H2O (0.028 g, 0.12 mmol, 0.52 equiv), MeOH (20 mL), H2O
(10 mL). Three hour reflux. Light gray solid (0.12 g, 0.097
1
mmol, 81% yield). H NMR (400 MHz, DMSO-d6) δ 75.34,
66.56, 44.74, 13.84, 11.95, 10.74. 19F NMR (376 MHz,
DMSO-d6) δ −58.23 (s, 12F, −CF3), −68.30 (d, J = 711 Hz,
12F, PF−6 ). HR-MS (ESI+) m/z calc’d for C46H26F12N6Ni2+
474.0685, found 474.0693 ([M - 2PF6]2+). Anal. Calc’d for
C46H26F24N6NiP2: C, 44.58; N, 6.78. Found: C, 44.65; N, 7.23.
[Ni(3-OMe)2][PF6]2. 3-OMe (0.10 g, 0.29 mmol), NiCl2·
6H2O (0.031 g, 0.13 mmol, 0.45 equiv), MeOH (20 mL), H2O
(10 mL). Three hour reflux. White solid (0.082 g, 0.080 mmol
1
61% yield). H NMR (400 MHz, DMSO-d6) δ 76.72, 69.06,
44.53, 13.75, 11.16, 11.00, 7.13, 4.24. 19F NMR (376 MHz,
DMSO-d6) δ −68.30 (d, J = 711 Hz, PF−6 ). HR-MS (ESI+)
m/z calc’d for C44H34N6NiO22+ 368.1043, found 368.1053 ([M
- 2PF6]2+). Anal. Calc’d for C44H34F12N6NiO2P2: C, 51.44; N,
8.18. Found: C, 51.79; N, 8.00.
[Zn(3,5-CF3)2][PF6]2. 3,5-CF3 (0.50 g, 1.1 mmol), ZnCl2
(0.071 g, 0.51 mmol, 0.45 equiv), MeOH (100 mL), H2O (50
mL). Three hour reflux. White solid (0.145 g, 0.116 mmol,
1
23% yield). H NMR (400 MHz, DMSO-d6) δ 8.71 (s, 2H,
H3′), 8.33 (d, J = 8.1 Hz, 2H), 8.21 (s, 2H, H2”’), 7.67 (s, 1H,
H4”’), 7.49 (t, J = 7.8 Hz, 2H), 7.13 (d, J = 5.1 Hz, 2H), 6.69
(t, J = 6.4 Hz, 2H). 19F NMR (376 MHz, DMSO-d6) δ
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[Ni(3-CF3)2][PF6]2. 3-CF3 (0.10 g, 0.27 mmol), NiCl2·
6H2O (0.03 g, 0.12 mmol, 0.46 equiv), MeOH (20 mL), H2O
(10 mL). One hour reflux. Light yellow solid (0.12 g, 0.109
mmol, 91% yield).
[Ni(4-OMe)2][PF6]2. 4-OMe (0.10 g, 0.29 mmol), NiCl2·
6H2O (0.33 g, 0.14 mmol, 0.47 equiv), MeOH (20 mL), H2O
(10 mL). Three hour reflux. Pale brown solid (0.11 g, 0.107
mmol, 76% yield).
[Zn(4-OMe)2][PF6]2. 4-OMe (0.11 g, 0.31 mmol), ZnCl2
(0.018 g, 0.13 mmol, 0.44 equiv), MeOH (20 mL), H2O (10
mL). Three hour reflux. White solid (0.11 g, 0.106 mmol, 82%
yield).
ACKNOWLEDGMENTS
■
The authors thank VMI for their financial support under the
Grants-In-Aid of Research Fund, the Summer Undergraduate
Research Institute, Wetmore Funds, and Jackson-Hope Funds,
as well as acknowledge the Virginia Equipment Trust Fund for
equipment purchases. We are perpetually grateful to all VMI
Chemistry private donors for their continued financial support
both for student stipends and consumable supplies. We are
especially grateful of Dr. William Hamilton (Washington and
Lee University) for performing combustion analysis.
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CF3,4-OMe, H, 19F, and 13C{1H} NMR spectra for
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Accession Codes
CCDC 2079232−2079251 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
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AUTHOR INFORMATION
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Corresponding Author
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Coudret, C.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L.
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Daniel P. Harrison − Virginia Military Institute, Department
of Chemistry, Lexington, Virginia 24450, United States;
orcid.org/0000-0003-1840-2561; Email: harrisondp@
vmi.edu
Authors
John C. Dickenson − Virginia Military Institute, Department
of Chemistry, Lexington, Virginia 24450, United States
MacKenzie E. Haley − Virginia Military Institute, Department
of Chemistry, Lexington, Virginia 24450, United States
Jacob T. Hyde − Virginia Military Institute, Department of
Chemistry, Lexington, Virginia 24450, United States
Zachary M. Reid − Virginia Military Institute, Department of
Chemistry, Lexington, Virginia 24450, United States
Travis J. Tarring − Virginia Military Institute, Department of
Chemistry, Lexington, Virginia 24450, United States
Diana A. Iovan − Virginia Tech, Department of Chemistry,
Blacksburg, Virginia 24060, United States; orcid.org/
0000-0001-9889-7183
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Notes
The authors declare no competing financial interest.
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