Bimetallic Complexes Supported by a Redox-Active Ligand with Fused Pincer-Type Coordination Sites

Article abstract of DOI:10.1021/acs.inorgchem.5b01380

The remarkable chemistry of mononuclear complexes featuring tridentate, meridionally chelating "pincer" ligands has stimulated the development of ligand frameworks containing multiple pincer sites. Here, the coordination chemistry of a novel pentadentate

Full text of DOI:10.1021/acs.inorgchem.5b01380

Article
pubs.acs.org/IC
Bimetallic Complexes Supported by a Redox-Active Ligand with
Fused Pincer-Type Coordination Sites
Denan Wang, Sergey V. Lindeman, and Adam T. Fiedler*
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201, United States
S
* Supporting Information
ABSTRACT: The remarkable chemistry of mononuclear complexes
featuring tridentate, meridionally chelating “pincer” ligands has stimulated
the development of ligand frameworks containing multiple pincer sites.
Here, the coordination chemistry of a novel pentadentate ligand (LN3O2
)
that provides two closely spaced NNO pincer-type compartments fused
together at a central diarylamido unit is described. The trianionic LN3O2
chelate supports homobimetallic structures in which each M(II) ion (M =
Co, Cu, Zn) is bound in a meridional fashion by the bridging diarylamido N
atom and O,N-donors of the salicyaldimine arms. The metal centers are also coordinated by a mono- or bidentate auxiliary ligand
(Laux), resulting in complexes with the general form [M2(LN3O2)(Laux)2]+ (where Laux = 1-methyl-benzimidazole (1MeBI), 2,2′-
bipyridine (bpy), 4,4′-dibromo-2,2′-bipyridine (bpyBr2), or (S)-2-(4-isopropyl-4,5-dihydrooxazolyl)pyridine (S-iPrOxPy)). The
fused nature of the NNO pincer sites results in short metal−metal distances ranging from 2.70 Å for [Co2(LN3O2) (bpy)2]+ to
3.28 Å for [Zn2(LN3O2) (bpy)2]+, as revealed by X-ray crystallography. The complexes possess C2 symmetry due to the twisting of
the aryl rings of the μ-NAr2 core; spectroscopic studies indicate that chiral Laux ligands, such as S-iPrOxPy, are capable of
controlling the helical sense of the LN3O2 scaffold. Since the four- or five-coordinate M(II) centers are linked solely by the amido
moiety, each features an open coordination site in the intermetallic region, allowing for the possibility of metal−metal
cooperativity in small-molecule activation. Indeed, the dicobalt(II) complex [Co2(LN3O2) (bpyBr2)2]+ reacts with O2 to yield a
dicobalt(III) species with a μ-1,2-peroxo ligand. The bpy-containing complexes exhibit rich electrochemical properties due to
multiple metal- and ligand-based redox events across a wide (3.0 V) potential window. Using electron paramagnetic resonance
(EPR) spectroscopy and density functional theory (DFT), it was determined that one-electron oxidation of [Co2(LN3O2
(bpy)2]+ results in formation of a S = 1/2 species with a LN3O2-based radical coupled to low-spin Co(II) centers.
)
exogenous ligands not connected to the pincer unit.9 In other
systems, the two metal centers are bridged by pyrazine,10 1,4-
phenylene,11 or ferrocene12 spacers, resulting in metal−metal
distances greater than 6 Å. Some groups have connected the
pincer compartments by more flexible spacers,13 thereby
allowing the metal centers to approach one another in space.
For example, the Ozerov group recently generated a series of
ligands in which two PNN pincer units are connected by an
alkyl spacer, (CH2)n (n = 2 or 4).14 These ligands were used to
prepare hydride-bridged palladium complexes with Pd−Pd
distances near 3 Å, as well as a complex featuring a metal−
metal bond (Pd−Pd distance of 2.56 Å).
As described in this manuscript, short intermetallic distances
can also be achieved by removing the spacer between the two
pincer sites entirely. In such “fused” ligands, the two pincer
coordination pockets share one of the donor arms, which then
serves as the single bridge between the metal centers. This
approach to bis(pincer) design has not been widely employed,
but Meyer and co-workers reported fused (or “two-in-one”)
PNN15 and NNN16 pincer ligands featuring a bridging
pyrazolate that provides one N-donor to each ferrous center
1. INTRODUCTION
Tridentate ligands that coordinate in a meridional fashion are
often called pincers due to their rigidity and tightly binding
nature.1 Pincer ligands have found application in nearly all areas
of inorganic chemistry, including transition-metal catalysis,2
sensors,3 and materials science.4 While considerable diversity
exists within this ever-expanding ligand family, a classic type of
pincer features a central diarylamido unit with two flanking
phosphine donors (PNP pincers).5 Several diarylamido-based
NNN pincers have also been generated, where the N-donor
arm is an imine, amine, or heterocyclic donor.6 Such ligands
have been used in the preparation of low-coordinate, yet highly
stable, transition-metal complexes capable of performing
diverse chemical transformations.
Given the remarkable utility of mononuclear pincer
complexes, there has been interest in developing binuclear
complexes that contain two pincer-type compartments. The
presence of two metal ions offers several catalytic advantages,
most crucially the possibility of cooperative action in substrate
binding/activation and the ability to perform multiple electron
transfers (assuming the metal centers are redox active). A few
“bis(pincer)” complexes have been prepared by the dimeriza-
tion of mononuclear species;7 in most of these cases, the metal
centers are doubly bridged by either the pincer arms8 or the
Received: June 19, 2015
© XXXX American Chemical Society
A
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(Figure 1a). Here, we expand upon the fused approach through
rings of the amido unit imparts rotational flexibility to the
pendant salicyaldimine arms.
the synthesis of the novel bis(pincer) ligand LN3O2, shown in
In this manuscript, we report the syntheses and X-ray
structural characterization of the homobimetallic complexes 1−
5 indicated in Chart 1. These complexes have the general
Chart 1
Figure 1. Complexes featuring fused (or “two-in-one”) pincer ligands:
(a) PNN bis(pincer) ligand recently reported by Meyer and co-
workers,15 and (b) the LN3O2 ligand described in this manuscript.
formula [M2(LN3O2)(Laux)2]+ (M = Co, Cu, Zn), where Laux
represents 1-methylbenzimidazole (1MeBI), 2,2′-bipyridine
(bpy), or 4,4-dibromo-2,2-bipyridine (bpyBr2). Chiral
[Zn2(LN3O2)]+ frameworks were prepared using the optically
active bidentate ligand (S)-2-(4-isopropyl-4,5-dihydro-
oxazolyl)pyridine (S-iPrOxPy; Chart 1). First-row transition
metals were selected because of their earth abundance, redox-
active nature (except for Zn), and proven ability to perform
small-molecule activation. The electrochemical properties of
2−5 were thoroughly examined with cyclic and square-wave
voltammetries. These bpy-containing complexes exhibit an
abundance of electrochemical features arising from both ligand-
and metal-based events; indeed, complexes 2 and 4 exhibit six
redox events over a potential range of nearly 3.0 V. The
electronic structures of [Co2(LN3O2) (bpy)2](ClO4) (4) and its
one-electron oxidized derivative (4ox) were examined with
spectroscopic and computational methods. These results
indicate that 4ox is a S = 1/2 species in which the μ-NAr2
unit of the LN3O2 ligand carries a large amount of unpaired spin
density. Finally, we demonstrate that [Co2(LN3O2) (bpyBr2)2]-
(ClO4) (5) reacts with O2 to yield a dicobalt(III) complex with
a bridging peroxo ligand, suggesting that these bimetallic
complexes are capable of small-molecule activation.
Figure 1b. Like conventional PNP and NNN pincers, the LN3O2
ligand contains a central diarylamido donor; however, the
presence of the two salicyaldimine chelates allows LN3O2 to
behave as a binucleating ligand with the diarylamido unit in a
bridging position. The resulting framework provides pincer-
type coordination to two metal centers in close proximity (3 Å
or less).
While not nearly as common as binucleating frameworks
with phenolate or pyrazolate bridges, ligands like LN3O2 that
feature a bridging diarylamido unit have yielded complexes with
attractive electronic and structural properties. For instance,
PNP ligands have been used to generate various bimetallic(I)
complexes with M2(μ-NAr2)2 diamond cores (M = Cu,17 Ni,18
and Co19). Although these complexes utilize PNP ligands, the
resulting four-coordinate M(I) centers exist in pseudotetrahe-
dral N2P2 environments instead of pincer-like sites. Spectro-
scopic and computational analysis revealed that the redox
chemistry occurs primarily at the bridging N atoms20a
finding consistent with previous studies of mononuclear
complexes with PNP pincer ligands.21 The “noninnocent”
nature of μ-NAr2 ligands is a potential asset in catalytic
processes requiring multiple electron transfers.22 Another
advantage of diarylamido ligands is their intrinsic chirality due
to the relative orientation of the aromatic rings, which gives rise
to atropisomers with C2 symmetry.14,23 Thus, diarylamido-
based ligands could lead to chiral bimetallic complexes for
enantioselective catalysis.
To date, the vast majority of bimetallic complexes with
bridging diarylamido ligands have featured M2(μ-NAr2)2
diamond cores.17−19,24 However, such structures are not ideal
for catalysis or small-molecule activation, as the steric bulk of
the four phenyl rings limits access to the metal centers. In
contrast, the LN3O2 ligand provides a more open framework that
preserves three vacant coordination sites on each metal center
in a meridional arrangement, similar to mononuclear pincer-
based complexes. These open coordination sites are available
for the binding of substrates and/or auxiliary ligands (Laux) with
advantageous structural or electrochemical properties, as
described below. Due to the presence of the salicyaldimine
chelates, LN3O2 bears resemblance to binucleating salen and
Schiff base ligands that have proven useful in catalysis and
materials chemistry.25 The variable dihedral angle between aryl
2. EXPERIMENTAL SECTION
Materials and Physical Methods. Unless otherwise noted, all
reagents and solvents were purchased from commercial sources and
used as received. Dichloromethane and acetonitrile were purified and
dried using a Vacuum Atmospheres solvent purification system. Due to
their air-sensitive nature, the dicobalt(II) complexes (4 and 5) were
handled under inert atmosphere using a Vacuum Atmospheres Omni-
Lab glovebox. Elemental analyses were performed at Midwest
Microlab, LLC, in Indianapolis, IN.
UV−vis absorption spectra were obtained with an Agilent 8453
diode array spectrometer, and CD spectra were recorded using a Jasco
J-715 spectropolarimeter. Infrared (IR) spectra of solid samples were
measured with a Thermo Scientific Nicolet iS5 FTIR spectrometer
equipped with the iD3 attenuated total reflectance accessory. 1H NMR
spectra were collected at room temperature with a Varian 400 MHz
spectrometer. EPR experiments were performed using a Bruker
ELEXSYS E600 equipped with an ER4415DM cavity, an Oxford
Instruments ITC503 temperature controller, and an ESR-900 He flow
cryostat. The program EasySpin4 was used to simulate the
experimental spectra.26 Electrochemical measurements were per-
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formed with an epsilon EC potentiostat (iBAS) under nitrogen
atmosphere at a scan rate of 100 mV/s with 0.1 M (NBu4)PF6 as the
electrolyte. A three-electrode cell containing a Ag/AgCl reference
electrode, a platinum auxiliary electrode, and a glassy carbon working
electrode was employed for voltammetric measurements. Under these
conditions, the ferrocene/ferrocenium (Fc+/0) couple has an E1/2 value
of +0.47 V in MeCN.
Bis(4-methyl-2-nitrophenyl)amine. This procedure was adapted
from previously published reports.27 A round-bottom flask equipped
with a stir bar was charged with a mixture of concentrated nitric acid
(70%; 10 mL) and glacial acetic acid (45 mL). The mixture was cooled
to 0 °C, and bis(4-methylphenyl)amine (5.00 g, 25.4 mmol, 1.0 equiv)
was added. The yellow mixture was stirred for 10 min in an ice/water
bath before dropwise addition of isoamyl nitrite (8.75 g, 75 mmol, 3.0
equiv) over the course of 5 min. The solution turned to dark green;
after stirring for an additional 10 min, the solution changed to an
orange color and a precipitate started to form. The orange precipitate
was collected via filtration, washed with diethyl ether, and dried under
vacuum (6.45 g, 22.5 mmol, 89% yield). 1H NMR (400 MHz, CDCl3)
δ: 2.37 (s, 6H, CH3), 7.32 (d, J = 8.5 Hz, 2H, ArH), 7.42 (d, J = 8.5
Hz, 2H, ArH), 7.99 (s, 2H, ArH), 10.80 (s, 1H, NH). 13C NMR (101
MHz, CDCl3) δ: 20.43, 119.73, 125.25, 126.33, 131.73, 135.19,
135.85.
Bis(4-methyl-2-aminophenyl)amine. A 250 mL pressure vessel
equipped with a stir bar was filled with solid bis(4-methyl-2-
nitrophenyl)amine (3.00 g, 10.45 mmol, 1.0 equiv), MeOH (100
mL), and 10% Pd/C (500 mg, 0.47 mmol, 0.0045 equiv). The flask
was pressurized with H2 gas (46 psi), and the mixture was stirred at 65
°C for 4 h, during which the bright orange solution became colorless.
The mixture was filtered through Celite and washed with cold MeOH,
and the solvent was removed under vacuum to yield a light purple oil.
The oil residue was triturated with Et2O (20 mL) to give a pale brown
powder, which was collected by filtration and dried under vacuum
(2.25 g, 9.93 mmol, 95% yield). 1H NMR (300 MHz, CDCl3) δ: 2.25
(s, 6H, CH3), 3.56 (s, 4H, NH2), 4.80 (s, 1H, NH), 6.54 (d, J = 7.9 Hz,
2H, ArH), 6.63−6.58 (m, 4H, ArH). 13C NMR (75 MHz, CDCl3) δ:
21.14, 109.99, 117.27, 120.49, 129.10, 133.11, 138.61.
1229.4 g mol−1): C, 60.56; H, 6.07; N, 7.97. Found: C, 60.35; H, 5.69;
N, 7.82. UV−vis [λmax, nm (ε, M−1 cm−1) in MeCN]: 310 (19 000),
424 (13 100). FTIR (cm−1; solid): 2953 (m), 2903 (w), 2864 (w),
1593 (m), 1523 (m), 1493 (m), 1456 (m), 1420 (m), 1358 (s), 1151
1
(s). 19F NMR (δ, CD3CN): −79.3 (OTf). H NMR (δ, CD3CN):
−30.6 (2H), 1.4 (18H), 2.2 (18H), 4.0 (6H), 6.2 (2H), 7.5 (2H), 8.8
(2H), 16.2 (2H), 22.7 (2H), 30.5 (2H), 40.2 (2H), 41.8 (6H), 51.7
(2H). μeff = 2.41 μB (Evans method).
[Cu2(LN3O2) (bpy)2](OTf) (2). The procedure was nearly identical to
the one used to prepare complex 1; the only difference was the
replacement of 1MeBI with bpy (31.2 mg, 2.0 equiv). Yield = 104 mg
(84%). Anal. Calcd for C65H70Cu2F3N7O5S (MW = 1245.4 g mol−1):
C, 62.68; H, 5.67; N, 7.87. Found: C, 62.46; H, 5.73; N, 7.85. UV−vis
[λmax, nm (ε, M−1 cm−1) in MeCN]: 298 (40 200), 430 (15 100), 495
(sh). FTIR (cm−1; solid): 2948 (m), 2904 (w), 2863 (w), 1593 (m),
1521 (m), 1489 (m), 1352 (s), 1152 (s). 19F NMR (δ, CD3CN):
1
−79.3 (OTf). H NMR (δ, CD3CN): −29.5 (2H), 1.4 (18H), 2.8
(18H), 8.9 (2H), 9.6 (2H), 10.1 (2H), 12.9 (4H), 13.3 (2H), 18.1
(2H), 25.9 (2H), 33.6 (2H), 36.0 (2H), 39.9 (6H), 45.1 (2H), 116.0
(2H). μeff = 2.36 μB (Evans method).
[Zn2(LN3O2) (bpy)2](OTf) (3). To a 25 mL flask were added the pro-
ligand H3LN3O2 (66.0 mg, 0.100 mmol), 2,2′-bipyridine (31.2 mg, 2.0
equiv), and Zn(OTf)2 (72.7 mg, 2.0 equiv). The components were
dissolved in a 1:1 mixture of MeCN:CH2Cl2 (5 mL) and stirred for 5
min. The addition of NEt3 (42 μL, 3.0 equiv) caused the solution to
turn to an orange color. The mixture was stirred overnight and filtered
through Celite, and the solvent was removed under vacuum. The
resulting powder was washed with Et2O (5 mL), dried under vacuum,
and dissolved in a 1:1 mixture of MeCN:MeOH (4 mL). Slow
evaporation over the course of 2 days provided orange crystals that
were collected via filtration, washed with Et2O, and dried under
vacuum. Yield = 96 mg (77%). Orange crystals, suitable for X-ray
diffraction, were grown from a concentrated 1:1 solution of
CH2Cl2:MeOH. Anal. Calcd for C65H70F3N7O5SZn2 (MW = 1249.1
g mol−1): C, 62.50; H, 5.65; N, 7.85. Found: C, 62.16; H, 5.60; N 7.76.
UV−vis [λmax, nm (ε, M−1 cm−1) in MeCN]: 425 (24 300), 460
(19 600). FTIR (cm−1; solid): 2949 (m), 2904 (w), 2865 (w), 1598
(m), 1523 (m), 1487 (m), 1379 (s), 1159 (s). 19F NMR (δ, CD3CN)
δ: −79.4 (OTf). 1H NMR (δ, CD3CN): 1.11 (s, 18H, C(CH3)3), 1.37
Pro-Ligand H3LN3O2. To a solution of bis(4-methyl-2-
aminophenyl)amine (454 mg, 2.0 mmol, 1.0 equiv) in MeOH (30
mL) was added 3,5-di-tert-butyl-2-hydroxybenzaldehyde (937.3 mg,
4.0 mmol, 2.0 equiv) and 5 drops of formic acid. The mixture was
heated overnight at 60 °C under an inert atmosphere, giving rise to a
bright yellow precipitate. The yellow solid was collected by filtration
and dried under vacuum to yield pure H3LN3O2 as a yellow powder
(1.20 g, 1.82 mmol, 91% yield). X-ray-quality crystals were obtained by
slow evaporation of a H3LN3O2 solution in a mixture of
CH2Cl2:CH3OH (9:1). Anal. Calcd for C44H57N3O2 (MW = 659.96
g mol−1): C, 80.08; H, 8.71; N, 6.37. Found: C, 80.32; H, 8.89; N,
6.27. 1H NMR (400 MHz, CDCl3) δ: 1.17 (s, 18H, C(CH3)3), 1.30 (s,
18H, C(CH3)3), 2.35 (s, 6H, CH3), 6.55 (s, 1H, NH), 6.99 (s, 2H,
ArH), 7.02 (d, J = 8.2 Hz, 2H, ArH), 7.17 (s, 2H, ArH), 7.36−7.30 (m,
4H, ArH), 8.64 (s, 2H, ArH), 13.26 (s, 2H, NC-H). 13C NMR (101
MHz, CDCl3) δ: 20.79, 29.13, 31.46, 34.11, 34.81, 116.46, 118.56,
119.27, 126.75, 127.78, 127.89, 130.44, 134.26, 136.82, 138.47, 140.22,
158.15, 163.94.
(s, 18H, C(CH3)3), 2.13 (s, 6H, CH3), 5.54 (d, J = 8.3 Hz, 2H, LN3O2
-
ArH), 6.17 (d, J = 8.3 Hz, 2H, LN3O2-ArH), 7.03 (s, 2H, LN3O2-ArH),
7.21 (dd, J = 8.1, 5.5 Hz, 2H, bpy-ArH), 7.26 (d, J = 2.7 Hz, 2H,
L
N3O2-ArH), 7.39 (d, J = 2.7 Hz, 2H, LN3O2-ArH), 7.45−7.51 (m, 2H,
bpy-H), 7.96 (d, J = 8.1 Hz, 2H, bpy-H), 7.99−8.07 (m, 8H, bpy-H),
8.48 (s, 2H, NC-H), 8.75 (d, J = 6.3 Hz, 2H, bpy-H).
[Co2(LN3O2) (bpy)2](ClO4) (4). The procedure was identical to the
one used to prepare complex 3, except for the substitution of
Co(ClO4)2·6H2O (73.2 mg, 2.0 equiv) for Zn(OTf)2. Yield = 85 mg
(71%). Brown crystals, suitable for X-ray diffraction, were grown from
a concentrated 1:1 solution of acetone:MeOH. Anal. Calcd for
C64H70ClCo2N7O6 (MW = 1186.6 g mol−1): C, 64.78; H, 5.95; N, 8.26.
Found: C, 62.37; H, 5.86; N, 7.91 (the discrepancies are due to small
amounts of [HNEt3]ClO4 salt, which persists even after multiple
recrystallizations). UV−vis [λmax, nm (ε, M−1 cm−1) in MeCN]: 440
(13 800), 590 (sh). FTIR (cm−1; solid): 2956 (m), 2904 (w), 2867
(w), 1587 (m), 1511 (m), 1441 (m), 1360 (s), 1162 (m). 1H NMR (δ,
CD3CN): 0.74 (s, 18H, C(CH3)3), 1.30 (s, 18H, C(CH3)3), 2.09 (s,
6H, CH3), 5.48 (d, J = 8.4 Hz, 2H, LN3O2-ArH), 6.13 (d, J = 8.4 Hz,
2H, LN3O2-ArH), 7.01 (s, 2H, LN3O2-ArH), 7.15 (d, J = 2.4 Hz, 2H,
[Cu2(LN3O2)(1MeBI)2](OTf) (1). The pro-ligand H3LN3O2 (66.0 mg,
0.100 mmol), 1MeBI (26.4 mg, 2.0 equiv), and Cu(OTf)2 (72.3 mg,
2.0 equiv) were added to a 25 mL flask containing MeCN (5 mL).
After stirring for 5 min, NEt3 (42 μL, 3.0 equiv) was added, causing
the solution to turn from brown to reddish brown. The mixture was
stirred for 2 h and then filtered through Celite. A small amount of
MeOH (1 mL) was slowly added to the filtrate, and the resulting
solution was allowed to slowly evaporate over the course of 2 days.
This process yielded orange-brown crystals that were collected,
washed with Et2O (3 mL), and dried under vacuum. Yield = 65 mg
(54%). X-ray-quality crystals were grown from a concentrated 1:1
solution of CH2Cl2:MeOH. The resulting structure revealed
uncoordinated MeOH molecules in the asymmetric unit, and
elemental analysis suggests that a small amount (∼1.0 equiv) remains
L
N3O2-ArH), 7.23 (d, J = 2.4 Hz, 2H, LN3O2-ArH), 7.34 (t, J = 6.0 Hz,
2H, bpy-H), 7.68 (t, J = 6.0 Hz, 2H, bpy-H), 7.73 (s, 2H, NC-H),
7.84 (d, J = 6.0 Hz, 2H, bpy-H), 7.96−8.12 (m, 6H, bpy-H), 8.18 (d, J
= 7.9 Hz, 2H, bpy-H), 10.23 (d, J = 6.0 Hz, 2H, bpy-H).
[Co2(LN3O2) (bpyBr2)2](ClO4) (5). This complex was prepared in the
same manner as complexes 3 and 4, with the exception that bpyBr2
(62.8 mg, 2.0 equiv) was used as the auxiliary ligand. Yield = 86 mg
(57%). The X-ray structure revealed two uncoordinated MeOH
molecules in the asymmetric unit. Anal. Calcd for
C64H66Br4ClCo2N7O6·2CH3OH (MW = 1566.3 g mol−1): C, 50.61;
H, 4.76; N, 6.26. Found: C, 48.98; H, 4.48; N, 6.31 (the discrepancies
after drying. Anal. Calcd for C61H70Cu2F3N7O5S·CH3OH (MW
=
C
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Figure 2. (a) Synthetic route for the pro-ligand H3LN3O2. (b) Molecular structure of H3LN3O2 determined by X-ray crystallography (50% probability
thermal ellipsoids).
are due to small amounts of [HNEt3]ClO4 salt). UV−vis [λmax, nm (ε,
M−1 cm−1) in MeCN]: 260 (30 000), 340 (14 900), 420 (8900), 580
(sh). FTIR (cm−1; solid): 3070 (w), 2948 (m), 2900 (w), 2863 (w),
1589 (m), 1543 (w), 1520 (m), 1492 (w), 1462 (m), 1397 (m), 1251
(m), 1170 (m). 1H NMR (δ, CD3CN): 0.77 (s, 18H, C(CH3)3), 1.31
Crystallographic Studies. X-ray diffraction data were collected at
100 K with an Oxford Diffraction SuperNova diffractometer equipped
with a 135 mm Atlas CCD detector and Cu Kα radiation source. The
resulting data were processed with the CrysAlis Pro program package
(Agilent Technologies, 2011). An absorption correction was
performed on the real crystal shape followed by an empirical multiscan
correction using SCALE3 ABSPACK routine. Structures were solved
using the SHELXS program and refined with the SHELXL program28
within the Olex2 crystallographic package.29 All non-hydrogen atoms
were refined with anisotropic displacement parameters. Hydrogen
atoms were generally positioned geometrically and refined using
appropriate geometric restrictions on bond lengths and bond angles
within a riding/rotating model, and the torsion angles of −CH3
hydrogens were optimized to better fit residual electron density. For
complexes 1, 3, and 5-O2, the unit cells contained large void spaces
filled with heavily disordered solvent, and exact localization of these
molecules was not feasible. The solvent-mask procedure implemented
in Olex2 was therefore applied to account for the contribution of these
solvent molecules to diffraction intensities. X-ray crystallographic
parameters are provided in Table S1, and experimental details are
available in the CIFs.
DFT Computations. DFT calculations were carried out using the
ORCA 2.9 software package developed by Dr. F. Neese (MPI for
Chemical Energy Conversion).30 When X-ray structures were not
available (4ox and 6), computational models were generated via
geometry optimizations that employed the Becke-Perdew (BP86)
functional.31 The computational models of 4 and 4ox omitted the tert-
butyl substituents of the phenolate donors. Calculations of the four
possible isomers of 6, however, involved the entire complex without
modification of the LN3O2 or S-iPrOxPy ligands. Once the optimized
models were obtained, molecular energies and electronic structure
parameters were calculated using Becke’s three-parameter hybrid
functional for exchange along with the Lee−Yang−Parr correlation
functional (B3LYP).32 All calculations utilized Ahlrichs’ valence triple-
ζ basis set (TZV) and TZV/J auxiliary basis set in conjunction with
polarization functions on all atoms.33 Solvent effects were calculated
using the conductor-like screening model (COSMO)34 with a
dielectric constant (ε) of 36.6 for MeCN. Exchange coupling constants
(J) were obtained using Noodleman’s broken symmetry approach (H
= −2JSA·SB).35 Isosurface plots of molecular orbitals were prepared
with Laaksonen’s gOpenMol program.36
(s, 18H, C(CH3)3), 2.17 (s, 6H, CH3), 5.62 (d, J = 8.2 Hz, 2H, LN3O2
-
ArH), 6.23 (d, J = 8.2 Hz, 2H, LN3O2-ArH), 7.10 (s, 2H, LN3O2-ArH),
7.17 (d, J = 2.5 Hz, 2H, LN3O2-ArH), 7.30 (d, J = 2.5 Hz, 2H, LN3O2
-
ArH), 7.57 (dd, J = 6.0, 1.9 Hz, 2H, bpy-H), 7.65 (d, J = 6.0 Hz, 2H,
bpy-H), 7.80 (s, 2H, NC-H), 7.92 (dd, J = 6.3, 1.9 Hz, 2H, bpy-H),
8.27 (d, J = 1.9 Hz, 2H, bpy-H), 8.46 (d, 2H, J = 1.9 Hz, bpy-H), 9.95
(d, J = 6.3 Hz, 2H, bpy-H).
[Co2(O2)(LN3O2) (bpyBr2)2](ClO4) (5-O2). Complex 5 was dissolved in
a 1:1 mixture of CH2Cl2:MeOH and exposed to air. Slow evaporation
over the course of several days provided dark brown crystals of both 5
and 5-O2 that were collected via filtration, washed with Et2O, and
dried under vacuum. Attempts to separate the two complexes were
unsuccessful. Regardless, the spectroscopic features of 5-O2 were
distinguished by comparison to data collected with pure samples of 5.
FTIR (cm−1; solid): 2948 (m), 2899 (w), 2863 (w), 1588 (m), 1543
(w), 1518 (m), 1493 (m), 1462 (m), 1398 (m), 1252 (m), 1170 (m),
1
1080 (s). H NMR (δ, CD3CN): 0.77 (s, 18H, C(CH3)3), 1.36 (s,
18H, C(CH3)3), 2.32 (s, 6H, CH3), 5.63 (d, J = 8.4 Hz, 2H, LN3O2
-
ArH), 6.55 (d, J = 8.4 Hz, 2H, LN3O2-ArH), 7.17 (d, J = 6.4 Hz, 2H,
bpy-H), 7.35 (d, J = 2.4 Hz, 2H, LN3O2-ArH), 7.37 (dd, J = 6.4, 2.0 Hz,
2H, bpy-H), 7.63 (d, J = 2.4 Hz, 2H, LN3O2-ArH), 7.70 (d, J = 2.0 Hz,
2H, bpy-H), 8.34 (dd, J = 6.1, 1.9 Hz, 2H, bpy-H), 8.37 (d, J = 1.9 Hz,
2H, bpy-H), 8.45 (s, 2H, LN3O2-ArH), 8.61 (s, 2H, NC-H), 9.00 (d,
J = 6.1 Hz 2H, bpy-H).
[Zn2(LN3O2)(S-iPrOxPy)2](OTf) (6). To a 25 mL flask were added the
pro-ligand H3LN3O2 (66.0 mg, 0.100 mmol), Zn(OTf)2 (72.7 mg, 2.0
equiv), and S-iPrOxPy (38 mg, 2.0 equiv). The components were
dissolved in a 3:1 mixture of MeCN:CH2Cl2 (8 mL) and stirred for 5
min. The addition of NEt3 (42 μL, 3.0 equiv) caused the yellow
solution to turn to a red-orange color. The mixture was stirred for 1 h
and filtered through Celite, and the solvent was removed under
vacuum. The resulting powder was washed with pentane (5 mL) and
dried under vacuum. Yield = 73.2 mg (56%). Anal. Calcd for
C67H82F3N7O7SZn2 (MW = 1317.2 g mol−1): C, 61.09; H, 6.27; N,
7.44. Found: C, 60.79; H, 5.81; N, 7.97 (the slight discrepancies are
due to small amounts of NEt3). UV−vis [λmax, nm (ε, M−1 cm−1) in
MeCN]: 420 (21 000), 460 (18 300). FTIR (cm−1; solid): 3055 (w),
2953 (m), 2906 (w), 2867 (w), 1653 (w), 1606 (m), 1591 (m), 1523
(m), 1487 (m), 1402 (m), 1236 (s), 1153 (s). 19F NMR (δ, CD3CN):
3. RESULTS AND DISCUSSION
3.A. Synthesis of H3LN3O2 and Bimetallic Complexes.
The pro-ligand H3LN3O2 is prepared by the route shown in
Figure 2. The final step in the synthesis is the condensation of
bis(2-amino-4-methylphenyl)amine with 2 equiv of 3,5-di-tert-
butylsalicylaldehyde. Formation of the two salicyaldimine units
required the presence of tert-butyl groups on the phenol ring.
Salicylaldehydes with less bulky substituents reacted instead
with the central diarylamine moiety to give a cyclized 2-
(benzimidazol-2-yl)phenol product (Figure S1). The identity of
H3LN3O2 was confirmed by 1H NMR and X-ray crystallography.
In the solid state, H3LN3O2 adopts a twisted conformation
1
−79.4. H NMR (δ, CD3CN): 0.52 (d, J = 6.8 Hz, 6H, CH(CH3)2),
0.59 (d, J = 6.8 Hz, 6H, CH(CH3)2), 1.11 (s, 18H, C(CH3)3), 1.36 (s,
18H, C(CH3)3), 1.56−1.62 (m, 2H, CH(CH3)2), 2.22 (s, 6H, CH3),
3.92−3.82 (m, 2H, oxazol-H), 4.48 (dd, J = 9.4, 6.9 Hz, 2H, oxazol-H),
4.65 (t, J = 9.4 Hz, 2H, oxazol-H), 6.80 (d, J = 8.2 Hz, 2H, LN3O2
-
ArH), 6.89 (d, J = 8.2 Hz, 2H, LN3O2-ArH), 6.93 (s, 2H, LN3O2-ArH),
7.12 (d, J = 2.7 Hz, 2H, LN3O2-ArH), 7.16 (dd, J = 7.9, 5.0 Hz, 2H, py-
H), 7.40 (d, J = 2.7 Hz, 2H, LN3O2-ArH), 7.57 (d, J = 7.9 Hz, 2H, py-
H), 7.98 (t, J = 7.9 Hz, 2H, py-H), 8.10 (s, 2H, NC-H), 8.58 (d, J =
5.0 Hz, 2H, py-H).
D
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featuring hydrogen bonds between phenol donors and imine
acceptors (Figure 2).
Table 1. Selected Bond Distances (Angstroms) and Angles
(degrees) for the Two Symmetry-Independent Units (A and
B) in the Crystal Structure of [Cu2(LN3O2)(1MeBI)2]OTf (1)
The bimetallic complexes 1−6 were prepared by the reaction
of H3LN3O2 with 2 equiv of the appropriate MX2 salt (M = Co,
Cu, Zn; X = OTf, ClO4) in the presence of NEt3, along with
addition of 2 equiv of the desired auxiliary ligand (1MeBI,
bpyR2, or S-iPrOxPy). The resulting complexes are soluble in
CH2Cl2 and polar aprotic solvents but insoluble in MeOH. All
of the complexes possess a dark orange-brown color due to an
absorption manifold with λmax near 420 nm (ε ≈ 104 M−1
cm−1). This feature is attributed to π−π* transitions of the
LN3O2 ligand based on its intensity and consistent presence
irrespective of metal ion or auxiliary ligand. As expected,
bond lengths
Cu1···Cu2
Cu1−O1
Cu1−N1
Cu1−N3
Cu1−N4 (1MeBI)
Cu1−O6 (MeOH)
Cu2−O2
Cu2−N2
Cu2−N3
A
B
3.0069(4)
1.908(2)
1.940(2)
2.049(2)
1.991(2)
2.537(2)
1.902(2)
1.920(2)
2.035(2)
1.987(2)
A
3.1202(4)
1.921(2)
1.950(2)
2.061(2)
2.006(2)
2.404(2)
1.898(2)
1.925(2)
2.039(2)
1.982(2)
B
1
dicopper complexes 1 and 2 give rise to H NMR spectra with
Cu2−N6 (1MeBI)
bond angles
broad, paramagnetically shifted peaks (Figure S2). Using the
Evans method, effective magnetic moments (μeff) of 2.38
0.03 μB were measured for 1 and 2 at room temperature; these
values are slightly less than the spin-only value of 2.56 μB
expected for a binuclear species with two uncoupled S = 1/2
Cu1−N3−Cu2
O1−Cu1−N1
O1−Cu1−N3
O1−Cu1−N4
N1−Cu1−N3
N1−Cu1−N4
N3−Cu1−N4
O2−Cu2−N2
O2−Cu2−N3
O2−Cu2−N6
N2−Cu2−N3
N2−Cu2−N6
N3−Cu2−N6
94.83(8)
92.55(7)
162.50(7)
88.53(7)
84.05(7)
175.12(8)
96.31(7)
94.05(7)
164.17(7)
89.15(7)
84.13(7)
166.02(8)
96.40(7)
66.3
99.10(8)
92.46(7)
162.62(7)
90.19(7)
84.20(7)
175.51(8)
94.32(7)
93.55(7)
167.41(7)
89.46(8)
84.46(7)
168.43(8)
94.96(8)
73.2
1
spins. In contrast, H NMR spectra of the dizinc and dicobalt
complexes (3−5) display sharp peaks with chemical shifts
indicative of diamagnetic ground states (Figure S3). The lack of
paramagnetism in the dicobalt complexes 4 and 5 is somewhat
surprising, and it suggests the presence of strong antiferro-
magnetic coupling between the Co2+ centersa matter that
will be examined below.
3.B. Solid-State Structures of Complexes 1−5. Dark
orange-brown crystals of the bimetallic complexes 1−5 suitable
for X-ray diffraction analysis were grown from concentrated 1:1
solutions of CH2Cl2 and MeOH (or 1:1 acetone:MeOH in the
case of 4). Attempts to generate X-ray-quality crystals of 6 were
unsuccessful. Selected bond lengths and angles are provided in
Tables 1 and 2, and the representative structures of
[Cu2(LN3O2)(1MeBI)2]+ (1+) and [Cu2(LN3O2) (bpy)2]+ (2+)
are shown in Figure 3. In each structure, the LN3O2 ligand
supports a bimetallic core in which the metal centers are solely
bridged by the central diarylamido group. For example, the unit
cell of 1 contains two symmetrically independent dicopper
complexes with Cu1···Cu2 separations (dCu−Cu) of 3.0069(4)
and 3.1202(4) (Table 1). The central [Cu2N]3+ unit exhibits a
Cu1−N3−Cu2 angle near 97°, giving rise to an intermetallic
“cleft”. As intended, the LN3O2 framework provides meridional
[N,N,O]2− coordination to both Cu2+ centers. The amido and
phenolate donors of the fused pincer-type sites are pulled back
slightly with O(1/2)−Cu(1/2)−N3 bond angles of 165 3°.
The Cu−O/N bond distances range from approximately 1.90 Å
for the phenolate donors (O1,O2) to 2.05 Å for the bridging
amido (N3) ligand. The additional coordination of two 1MeBI
auxiliary ligands to each Cu2+ ion results in distorted square-
planar geometries, although Cu1 is also weakly bound to a
MeOH solvate (the Cu1−O6 distance is greater than 2.40 Å).
The planes of the 1MeBI ligands are oriented nearly
perpendicular to the square-planar [CuN3O] units, and the
1MeBI phenyl rings are positioned parallel to the imine groups
on the inside of the cleft (Figure 3a).
a
twist angle
a
The twist angle refers to the angle between the planes of the aryl
rings of the central amido unit.
possess idealized C2 symmetry and exist as racemic mixtures of
(M)- and (P)-enantiomers, as illustrated in Figure 4 [Note: The
(P)-enantiomers of 1 and 2 are displayed in Figure 3]. In the
next section, we will demonstrate that use of a chiral auxiliary
ligand can force the LN3O2 ligand to favor one conformation
over the other, resulting in a single diastereomeric product.
The three bpy-containing complexes 2−4 (Chart 1) yield
quasi-isomorphous crystals in the monoclinic P21/c space
group (Table S1). The structure of [Cu2(LN3O2) (bpy)2]+ (2+),
shown in Figure 3b, is representative of the series. In each
complex, the two metal ions occupy equivalent binding sites
defined by the pincer-type N,N,O-chelates of the LN3O2 ligand,
which adopts the same C2-symmetric (“twisted”) conformation
described above. Coordination of the bidentate bpy ligands
results in five-coordinate M2+ ions with distorted square-
pyramidal (4) or trigonal-bipyramidal (3) geometries, as
indicated by τ values37 in Table 2. The presence of the fifth
donor causes the M2+ ions to move out of the plane defined by
the meridional LN3O2 chelate by amounts ranging from 0.36 (4)
to 0.80 Å (3). Each bpy ligand places one pyridyl donor trans to
the imine N atom (N1 or N2), while the other is located
outside the cleft in a position opposite the intermetallic bond
vector. Unlike the 1MeBI ligands in 1, the bpy ligands in 2−4
do not block access to the space between the metal ions, thus
permitting the binding of small molecules like O2 (vide infra).
Table 2 reveals several trends in metric parameters across the
2−4 series. The M−LN/O bond distances and intermetallic
separations are strongly dependent on metal ion identity, both
following the general order of Zn > Cu > Co. Of particular note
is the observed Co···Co distance (dCo−Co) of 2.701 Å, which is
The meridional binding mode of the LN3O2 framework causes
each half of the ligand to adopt an orientation in which the
salicyaldimine unit is roughly coplanar with the adjacent
arylamido ring. However, these planar halves of the LN3O2
ligand are rotated relative to one another due to the twisting of
the diarylamido unit. The solid-state structures of 1−5 revealed
“twist angles” between 60° and 73° for the aryl rings bonded to
N3 (Tables 1 and 2). Therefore, the bimetallic complexes
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Table 2. Selected Bond Distances (Angstroms) and Angles (degrees) Obtained from the Crystal Structures of Complexes 2−4
complex 2
complex 3
complex 4
bond distances
M1···M2
Cu1
Cu2
Zn1
Zn2
Co1
Co2
2.9667(5)
1.924(2)
1.940(2)
2.063(2)
2.201(2)
2.010(2)
2.028
3.2820(5)
1.952(1)
2.054(1)
2.086(1)
2.100(1)
2.116(1)
2.062
2.7010(5)
1.899(2)
1.889(2)
1.966(2)
2.064(2)
1.942(2)
1.952
M1−O1/M2−O2
M1−N1/M2−N2
M1−N3/M2−N3
M1−N4/M2−N6 (bpy)
M1−N5/M2−N7 (bpy)
M−LN/O (ave)
1.921(2)
1.940(2)
2.052(2)
2.228(2)
2.008(2)
2.030
1.956(1)
2.058(1)
2.103(1)
2.100(1)
2.118(1)
2.067
1.895(2)
1.888(2)
1.964(2)
2.059(2)
1.943(2)
1.950
complex 2
complex 3
complex 4
bond angles
Cu1
Cu2
Zn1
Zn2
Co1
Co2
M1−N3−M2
O1/O2−M−N1/N2
O1/O2−M−N3
O1/O2−M−N4/N6
O1/O2−M−N5/N7
N1/N2−M−N3
N1/N2−M−N4/N6
N1/N2−M−N5/N7
N3−M−N4/N6
92.28(8)
92.69(8)
146.38(8)
114.49(8)
91.23(8)
83.05(8)
93.85(8)
171.34(9)
99.09(8)
97.87(8)
77.50(8)
0.42
103.15(6)
89.07(5)
140.53(5)
114.66(5)
90.43(5)
81.72(5)
97.25(5)
174.87(5)
104.57(5)
101.83(5)
78.33(5)
0.57
86.85(8)
93.52(8)
158.41(7)
106.01(7)
84.38(7)
85.02(8)
91.98(8)
171.85(8)
95.57(8)
99.83(8)
81.08(8)
0.22
92.58(9)
152.33(8)
112.33(8)
88.30(8)
84.09(9)
94.33(8)
171.63(9)
95.33(8)
98.93(8)
77.66(8)
0.32
88.52(5)
133.17(5)
118.85(5)
94.39(5)
80.43(5)
97.34(5)
175.18(5)
107.69(5)
100.25(5)
77.89(5)
0.70
93.92(7)
156.69(7)
105.79(7)
86.33(7)
84.76(7)
91.57(8)
172.13(8)
97.51(8)
98.11(7)
80.81(8)
0.26
N3−M−N5/N7
N4/N6−M−N5/N7
a
τ value
b
twist angle
64.0
67.6
61.7
a
b
For definition of the τ value, see ref 37. The twist angle refers to the angle between the planes of the aryl rings of the central amido unit.
that nearly all di- and polycobalt complexes with dCo−Co < 2.8 Å
possess multiple bridging groups. While it is not proper to
invoke the existence of a Co−Co bond in 4, since the
intermetallic distance exceeds the sum of van der Waals radius
of Co (2.486 Å), this feature points to strong electronic
interactions between the Co2+ centers. In addition, the average
Co−LN/O distance of 1.951 Å in 4 is unusually small. High-spin
Co2+ complexes with N4O ligand sets typically exhibit average
Co−LN/O lengths of 2.07 0.05 Å, while the handful of low-
spin [CoN4O] structures in the literature feature average Co−
L
N/O bonds of 1.95 0.05 Å.38 Thus, the crystallographic data
indicate that 4 consists of two low-spin Co2+ centers in square-
pyramidal environments. The electronic structure of this
complex is described in more detail in the DFT section below.
Not surprisingly, the solid-state structures of complex 4 and
its bpyBr2-containing congener (5) are quite similar, with one
exception: the addition of bromine substituents causes a
modest lengthening of the Co···Co distance from 2.701 to
2.837 Å, with a concomitant increase in the Co1−N3−Co2
bond angle (Table S2). Otherwise, the Co−N/O bond lengths
in 4 and 5 differ by less than 0.025 Å, indicating that both
complexes feature low-spin Co2+ centers.
Figure 3. Thermal ellipsoid plots (40% probability) derived from the
X-ray crystal structures of complexes 1 (a) and 2 (b). Hydrogen
atoms, counteranions, and tert-butyl substituents of the phenolate
donors have been omitted for the sake of clarity.
3.C. Formation of the Chiral Complex [Zn2(P-LN3O2)-
(S-iPrOxPy)2]OTf (6). There has been a long-standing interest
in developing chiral bimetallic complexes for use in asymmetric
catalysis and supramolecular chemistry.39 As noted above, the
bimetallic complexes 1−5 possess helical chirality due to the C2
symmetry imposed by the LN3O2 framework (Figure 4). We
were curious whether the use of a chiral auxiliary ligand would
bias the helical sense of the LN3O2 ligand, thereby yielding a
single diastereomeric product. To this end, the reaction of
Zn(OTf)2 and H3LN2O3 with the auxiliary ligand S-iPrOxPy
(Chart 1) was performed in the presence of base. The 1H NMR
spectrum of the resulting product (6), shown in Figure 5,
Figure 4. Illustration of the two possible orientations of the C2-
symmetric LN3O2 ligand in complexes 1−6. The designations of the
atropisomers (M and P) were based on rules developed for binaphthyl
systems.
remarkably short given the presence of only one bridging
ligand. A search of the Cambridge Structural Database found
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Figure 6. Depictions of the cis-(P,S,S) diastereomer of complex 6.
groups of S-iPrOxPy and the LN3O2 ligand. Relative to cis-(P,S,S),
the cis-(M,S,S) diastereomer is higher in energy by 6.4 kcal/mol
because the isopropyl groups are directed toward the central
aryl rings. The trans-(M,S,S) and trans-(P,S,S) structures are
even more unfavorable energetically (7.0 and 9.9 kcal/mol,
respectively), as the isopropyl groups sterically clash with either
the phenyl rings of the diarylamido unit (M-isomer) or the tert-
butyl substituents of the phenolate donors (P-isomer). These
DFT results suggest that the major isomer of 6, observed
spectroscopically in solution, corresponds to the cis-(P,S,S)
diasteromer. On the basis of the computed energy differences,
we would not expect to detect the minor isomer at room
temperature. The fact that the minor cis-(M,S,S) diastereomer
Figure 5. (Top) Aromatic region of the 1H NMR spectrum of
complex 6 in MeCN-d3. Peaks are assigned to either the pyridyl (py)
moiety of S-iPrOxPy or the LN3O2 ligand based on splitting patterns and
comparison to data collected for H3LN3O2 and 3. Each of the 10
assigned features integrates to two H atoms. Ill-defined peaks marked
with asterisks arise from the minor diastereomeric product. (Bottom)
Absorption and CD spectra of 6 collected at room temperature in
MeCN.
1
is observed in the H NMR spectrum of 6 (Figure 5) suggests
displays only one set of well-resolved peaks arising from the
LN3O2 and S-iPrOxPy ligands, indicating that the material largely
consists of a single diastereomer. However, a number of weak
and poorly resolved features are also apparent (indicated by the
asterisks in Figure 5a); if these are assigned to the minor
diastereomer then relative peak heights suggest that at least
85% of 6 exists as the major product. Further evidence of
diastereomeric excess is provided by circular dichroism (CD)
spectroscopy. As shown in Figure 5b, the CD spectrum of 6
exhibits positive and negative bands at 470 and 380 nm,
respectively, corresponding to the two overlapping bands in the
that the M- and P-isomers cannot easily interconvert to yield
the thermodynamically favored product, and thus, the product
ratio is also affected by kinetic factors.
3.D. Electrochemical Studies. Voltammetric methods
were used to examine the electrochemical properties of
complexes 2−5 in MeCN solutions with 0.1 M (NBu4)PF6
as the supporting electrolyte and scan rates of 100 mV/s. The
reported potentials are relative to the Fc/Fc+ couple. The cyclic
and square-wave voltammograms (CV and SWV) of the
dicopper complex 2, shown in Figure 7, reveal three redox
features at negative potentials of −1.40, −1.83, and −2.06 V.
The two lowest potential events are quasi-reversible, whereas
the peak at −1.40 V is irreversible. The SWV of the analogous
dicobalt complex (4) displays three peaks at very similar
potentials of −1.42, −1.81, and −2.04 V. Assignment of these
features to either metal- or ligand-based reductions is aided by
comparison with data collected for 3, which contains redox-
inactive Zn2+ ions. The voltammogram of 3 retains the two
lowest potential features at −1.95 and −2.08 V; however, the
first reduction wave is absent (Figure 7). Therefore, we can
confidently assign the two events at E < −1.7 V to reduction of
the bpy auxiliary ligands, while the event near −1.4 V is
absorption spectrum. Since these features arise from LN3O2
-
based transitions (vide supra), we can conclude that the LN3O2
ligand largely exists in a single helical conformation. As
expected, the CD spectrum of the bpy-containing dizinc(II)
analog (3) is featureless across the UV−vis region (Figure S4)
because the complex exists as a racemic mixture of M- and P-
enantiomers.
In complex 6, the oxazoline ring of the asymmetric S-iPrOxPy
ligand can be positioned either cis or trans to the imine donors
(N1 and N2). The combination of geometric and stereo-
isomerism gives rise to four possible C2-symmetric structures:
trans-(M,S,S), cis-(M,S,S), trans-(P,S,S), and cis-(P,S,S). The
energy of each isomer was computed using DFT, and the most
stable structure was found to be cis-(P,S,S), shown in Figure 6.
This isomer minimizes steric interactions between the isopropyl
4+
4+
attributed to reduction of the Cu2 and Co2 units to yield
mixed-valent species. Electrochemical data obtained for the
bpyBr2-containing dicobalt complex (5) provide further
confirmation of these assignments. Relative to 4, the two
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is found in the CV and SWV data of the dizinc(II) analog (3),
which display an ill-defined event in the same region despite the
absence of redox-active metal ions. Previous studies of related
mononuclear complexes have detected phenolate oxidations in
45
the range of 0.5−1.5 V versus Fc+/0
,
although it is often
difficult to distinguish between ligand- and metal-based events
due to the high covalency of metal−phenolate bonds.
3.E. Spectroscopic and Computational Studies of 4
and 4ox. Density functional theory (DFT) was employed to
examine the unusual electronic properties of [Co2(LN3O2
)
(bpy)2]+ (4+), namely, its diamagnetism and short Co−Co
distance. These calculations employed the hybrid B3LYP
functional and crystallographically determined structure,
although the tert-butyl substituents of the phenolate rings
were replaced with H atoms. The proper wave function for the
S = 0 ground state was obtained using the broken-symmetry
(BS-DFT) approach pioneered by Noodleman and others.46
The BS-DFT calculations revealed strong antiferromagnetic
(AF) coupling between the low-spin Co2+ centers, with a
computed exchange coupling constant (J) of −1050 cm−1
(based on the Yamaguchi definition47 of H = −2JSA·SB). The
AF coupling arises from direct overlap of the 3d(z2)-based
singly occupied molecular orbitals (SOMOs) localized on each
Co2+ center (the local z axes are directed along the axial Co−N
bonds; see Figure S5). The overlap integral (S) for these two
magnetic orbitals with opposite spin is 0.29, consistent with the
presence of a partial Co−Co bond in 4. The magnitude of the
computed J value ensures that the triplet state is not accessible
at room temperature.
Figure 7. Cyclic voltammograms (solid lines) of complexes 2−5
collected in MeCN with 0.1 M (NBu4)PF6 as the supporting
electrolyte. Corresponding square-wave voltammograms are indicated
by the dashed lines. All scan rates were 100 mV/s. Data in the high-
and low-potential regions were generally collected in separate scans.
Two CVs with different sweep widths are provided for complex 2.
On the basis of the electrochemical results presented above,
we sought to generate the oxidized form of complex 4 via
chemical means. Treatment of 4 with 1 equiv of 1′-
acetylferrocenium (AcFc; E = 0.27 V) in MeCN causes the
ligand-based absorption band near 400 nm to red shift and
decrease in intensity (Figure 8; inset), indicative of a change in
the π system of the LN3O2 ligand. The X-band EPR spectrum of
the one-electron-oxidized species (4ox), shown in Figure 8,
consists of a broad derivative feature centered at g = 2.02. The
presence of 59Co hyperfine splitting at both high and low fields
is particularly evident in the second-harmonic spectrum. The
data is nicely simulated (Figure 8) with the following spin-
Hamiltonian parameters: a pseudoaxial g tensor (gx,y,z = 2.059,
2.037, 1.995) and hyperfine coupling constants of Ax,y,z = 23, 21
and 7.1 G for both Co ions. Low-spin, five-coordinate Co2+
centers typically display Amax values between 80 and 120 G and
gx values near 2.40.48 In contrast, the modest g anisotropy and
small A values of 4ox more closely resemble the EPR parameters
of mononuclear Co/O2 adducts (gx,y = 2.08 and Amax ≈ 20 G),
where the unpaired spin largely resides on the superoxo
ligand.48 Therefore, the EPR data provide further evidence that
one-electron oxidation of 4 generates a ligand-based radical,
although the unpaired spin is partially delocalized over the two
Co ions.
lowest potentials peaks of 5 are shifted positively by ∼0.23 V,
reflecting the electron-withdrawing capacity of the 4-Br
substituents. In contrast, the metal-based peak experiences a
much smaller shift from −1.42 to −1.33 V. The redox-active
nature of bpy ligands has been well-established in numerous
studies,40 including recent efforts by the Wieghardt group.41,42
The homoleptic [Fe(bpy)3]2+ complex, for example, exhibits
sequential reductions of the three bpy ligands at potentials of
−1.66, −1.94, and −2.10 V.41 The corresponding potentials in
2−4 are more negative than those reported for [Fe(bpy)3]n by
100−150 mV, likely due to the trianionic nature of the LN3O2
framework.
To more positive potentials, all four complexes exhibit a
feature between 0.07 and 0.19 V. While this redox event is
quasi-reversible for 2 and 4 (ΔE = 83 and 115 mV,
respectively), it is irreversible in the cases of 3 and 5, as
evident in the diminished intensity of the corresponding peaks
in the SWV (Figure 7). Given its presence across the 2−5
series, it is logical to attribute this feature to a ligand-based
event. Indeed, a survey of the literature revealed that the
diarylamido moieties of PNP pincer ligands are oxidized
between +0.32 and −0.34 V,18,21b,43 and the di(2-
pyrazolylaryl)amido pincers of Gardinier and co-workers
undergo oxidation near 0.0 V.6e,44 The common feature near
0.1 V in complexes 2−5 is therefore assigned to oxidation of
the μ-NAr2 unit of the LN3O2 ligand. Evidence for formation of
a ligand-based radical upon one-electron oxidation of 4 is
provided in the EPR and DFT studies described in section 3.E.
In addition, complexes 2 and 4 exhibit two closely spaced
waves centered near 0.90 V (Figure 7). These features
correspond to successive oxidations of the divalent metal
ions, although the processes likely involve partial oxidation of
the phenolate donors as well. Support for the latter conclusion
The nature of the LN3O2-based radical was probed with BS-
DFT calculations. Because a crystal structure of 4ox is not
available, a computational model was obtained via geometry
optimization. Comparison of Mulliken spin populations
indicate that one-electron oxidation of 4 to 4ox causes a
dramatic increase in the amount of unpaired spin density on the
LN3O2 ligand (from 0.04 to 0.74 spins), while the spin of the Co
centers remains nearly constant. The 4ox model contains three
unpaired electrons: two are localized in Co(dz2)-based MOs,
while the third is primarily localized on the central μ-NAr2 unit
H
DOI: 10.1021/acs.inorgchem.5b01380
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 9. Thermal ellipsoid plot (30% probability) derived from the
X-ray crystal structure of complex 5-O2. Hydrogen atoms, counter-
anions, and tert-butyl substituents of the phenolate donors have been
omitted for the sake of clarity.
Figure 8. X-band EPR spectrum of 4ox (black, solid) in frozen MeCN
at 77 K. The presence of 59Co hyperfine splitting is clearly apparent in
the corresponding second-harmonic data (top). Simulated spectra
of the electrochemical data presented above, one might have
assumed that O2 binding would result in oxidation of the LN3O2
ligand. It appears that the increase in coordination number
from 5 to 6, coupled with the dianionic nature of the peroxide
ligand, suppresses the redox potentials of the Co centers
relative to LN3O2. These factors favor metal-centered oxidation
over ligand-based oxidation in formation of the dicobalt-peroxo
complex.
(red, dashed) were obtained with the following parameters: gx,y,z
=
2.059, 2.037, 1.995; Ax,y,z = 23, 21, and 7.1 G; mwFreq = 9.434 GHz.
(Inset) Absorption spectra of 4 (blue, solid) and 4ox (black, dashed) in
MeCN (conc. = 0.1 mM).
of the LN3O2 ligand. The contour plot of the LN3O2-based
SOMO, shown in Figure S6, reveals overlap between the 2pz
orbital of the bridging N atom (N3) and Co 3d orbitals,
accounting for the observable 59Co hyperfine splitting in the
EPR spectrum of 4ox. The DFT calculations are therefore
4. CONCLUSIONS
As demonstrated in this manuscript, the easily prepared LN3O2
ligand is capable of supporting homobimetallic frameworks (M
= Co, Cu, Zn) with adjacent pincer-type compartments
consisting of a bridging diarylamido group and salicyaldimine
chelates (Figure 1). The “fused” nature of the pincer sites
results in short intermetallic distances between 2.7 and 3.3 Å, as
determined by X-ray crystallography. While several complexes
with [M2(μ-NAr2)2] cores have been reported in the literature,
complexes 1−6 are rather unique in containing only a single
diarylamido bridge.49 Because of this, the unsaturated metal
centers are capable of binding auxiliary ligands, such as 1MeBI,
bpyR2, and S-iPrOxPy. These auxiliary ligands impart additional
features to the bimetallic complexes that may prove useful in
future applications; for example, the noninnocent bpyR2 ligands
in 2−5 account for two redox events at low potentials, while
the optically active S-iPrOxPy ligand compels the C2-symmetric
structures to favor the P-configuration. Thus, the LN3O2 ligand
provides a versatile platform for the synthesis of bimetallic
complexes with tunable electronic and structural properties.
Electrochemical studies of the bpy-containing complexes 2
and 4 found six redox couples over a range of 3.0 V arising from
both metal- and ligand-based events. The one-electron
oxidation of 4 near 0.1 V triggers formation of a LN3O2-based
radical localized on the diarylamido donor, as indicated by EPR
and DFT studies of 4ox. This finding is consistent with previous
studies of [M2(μ-NAr2)2] complexes. The ability to perform
several electron transfers is critical for synthetic catalysts
involved in small-molecule activation, such as the reduction of
O2 (to H2O) and H+ (to H2). Redox-active ligands, like bpy
and LN3O2 in complexes 2−5, can serve as electron reservoirs
for multielectron transformations. Most significantly, the
consistent with the formulation of 4ox as [Co2+2(LN3O2,•
)
(bpy)2]2+, in agreement with the electrochemical and
spectroscopic data already presented.
3.F. Reactivity of Complex 5 with O2. If exposed to air,
solutions of 5 in 1:1 CH2Cl2:MeOH provide dark-brown
crystals with unit cell parameters distinct from those
determined for anaerobically grown crystals (Table S1). X-ray
diffraction analysis determined that the aerobic crystals consist
of [Co2(O2)(LN3O2) (bpy)2]ClO4 (5-O2), where a diatomic O2
ligand bridges in a μ-1,2-fashion between the six-coordinate
cobalt centers (Figure 9). The O3−O4 distance of 1.372(6) Å
identifies the bridging ligand as a peroxide (O22−) moiety. The
[Co2O2] unit adopts a twisted orientation with a Co−O−O−
Co dihedral angle of 56.3° and Co1···Co2 distance of 3.253(1)
Åconsiderably longer than the dCo−Co value of 2.70 Å found
for 5. Significantly, the 5-O2 structure proves that small
molecules are able to access the open coordination sites within
the intermetallic cleft, despite the steric bulk of the nearby tert-
butyl substituents of the phenolate donors. Moreover, the
dramatic 0.55 Å increase in intermetallic separation upon O2
binding highlights the structural pliability of the LN3O2 scaffold.
Comparison of metric parameters indicates that the
conversion of 5 → 5-O2 involves oxidation of the low-spin
Co2+ centers to Co3+. This conclusion is evident in the
shortening of the axial Co1−N4 and Co2−N6 bonds by ∼0.09
Å due to the transfer of two Co(dz2)-based electrons to O2
(Table S2). In contrast, O2 binding does not cause significant
changes in the O−C, N−C, and C−C bond lengths of the
LN3O2 ligand, indicating a lack of radical character. On the basis
I
DOI: 10.1021/acs.inorgchem.5b01380
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Inorganic Chemistry
Article
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01380.
■
S
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1H NMR spectra of complexes 1−5, absorption and CD
spectra of 3, metric parameters for 5 and 5-O2, and
contour plot of the singly occupied molecular orbital
(SOMO) of 4ox (PDF)
Crystallographic data in CIF format (CIF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: adam.fiedler@marquette.edu. Fax: 414-288-7066.
Notes
■
(14) Herbert, D. E.; Ozerov, O. V. Organometallics 2011, 30, 6641−
6654.
(15) Samanta, S.; Demesko, S.; Dechert, S.; Meyer, F. Angew. Chem.
2015, 54, 583−587.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was generously supported by Marquette
University and the U.S. National Science Foundation (NSF)
(CHE-1056845). We thank Dr. Thomas Brunold for allowing
us to collect CD spectra in his laboratory and Dr. James R.
Gardinier for helpful discussions.
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DOI: 10.1021/acs.inorgchem.5b01380
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