Three oxidation states of the bis(3,5-di-tert-butyl-2-phenolato)azanido pincer ligand on chromium(III)

Article abstract of DOI:10.1016/j.poly.2017.08.043

Three monomeric complexes of chromium(III) containing the redox-active ligand derived from bis(3,5-di-tert-butyl-2-phenol)amine, [ONO^cat]H₃, were prepared and fully characterized. The potassium salt of [ONO^q]^1? reacted with CrCl₃(THF)₃ to afford [ONO^q]CrCl₂(THF) (1); whereas, the trilithium salt of [ONO^cat]^3? reacted with CrCl₃(THF)₃ in the presence of pyridine to afford [ONO^cat]Cr(py)₃ (3). The three electron series was completed by reacting 1 with K[bpy^[rad]] to afford [ONO^sq[rad]]CrCl(bpy) (2). All three members of the series were characterized by structural, spectroscopic, and electrochemical methods. Attempts to effect the two-electron reduction of 1 resulted in the formation of a bimetallic chromium complex, {[ONO^cat]Cr(py)₂}₂ (4), which was also structurally characterized. The reaction of 1 with aryl transfer reagents resulted in a mixture of one- and two-electron reactivity, comprising the expulsion of either phenyl radical or biphenyl from the chromium coordination sphere.

Full text of DOI:10.1016/j.poly.2017.08.043

Polyhedron xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Three oxidation states of the bis(3,5-di-tert-butyl-2-phenolato)azanido
pincer ligand on chromium(III)
⇑
Aaron M. Hollas, Joseph W. Ziller, Alan F. Heyduk
Department of Chemistry, University of California, Irvine, CA 92697-2025, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Three monomeric complexes of chromium(III) containing the redox-active ligand derived from bis(3,5-
di-tert-butyl-2-phenol)amine, [ONOcat]H3, were prepared and fully characterized. The potassium salt of
Received 27 July 2017
Accepted 30 August 2017
Available online xxxx
[ONOq]1ꢀ reacted with CrCl3(THF)3 to afford [ONOq]CrCl2(THF) (1); whereas, the trilithium salt of
3ꢀ
[ONOcat
]
reacted with CrCl3(THF)3 in the presence of pyridine to afford [ONOcat]Cr(py)3 (3). The three
electron series was completed by reacting 1 with K[bpyÅ] to afford [ONOsqÅ]CrCl(bpy) (2). All three mem-
bers of the series were characterized by structural, spectroscopic, and electrochemical methods. Attempts
to effect the two-electron reduction of 1 resulted in the formation of a bimetallic chromium complex,
{[ONOcat]Cr(py)2}2 (4), which was also structurally characterized. The reaction of 1 with aryl transfer
reagents resulted in a mixture of one- and two-electron reactivity, comprising the expulsion of either
phenyl radical or biphenyl from the chromium coordination sphere.
Keywords:
Pincer ligand
Redox-active ligand
Non-innocent ligand
Chromium
Redox chemistry
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
the chromium derivative was only recently reported in the litera-
ture [25]. In the solid state, the experimental data suggested that
Chromium complexes of redox-active ligands have a rich his-
tory, dating back to the discovery of ligand non-innocence, with
the first reports being of chromium dithiolene complexes [1]. Sub-
sequent studies detailed a broad family of substituted tris(dithi-
olene)chromium complexes and their electron transfer series,
with the data supporting the predominance of chromium(III) metal
centers [2–11]. The stability of these dithiolene complexes, along
with their rich electrochemical and magnetic properties, prompted
intense research into the coordination chemistry of chromium(III)
with redox-active ligands, with scope expanded to cover all major
classes of redox-active ligands including bipyridine [12–14],
the homoleptic chromium complex was best described as a CrIV[-
ONOsqÅ
]
species, which behaved as a CrIII[ONOsqÅ][ONOq] complex
2
in solution.
Our own research interests lie in the application of redox-active
ligands to multi-electron reactivity. Whereas homoleptic com-
plexes such as those discussed above manifest interesting elec-
tronic and magnetic properties, in general their saturated
coordination spheres preclude small molecule activation reactions.
With this in mind, we set out to develop the coordination chem-
istry of chromium containing a single [ONO] ligand, and to deter-
mine the accessibility of such complexes with different ligand
oxidation states. Herein we report the synthesis of monomeric
chromium(III) complexes containing the [ONO] ligand in all three
established oxidation states along with preliminary reactivity
studies.
phenanthrene [12,15–17],
a-diimine [18], catechol [10,19–22],
amidophenol [23], and o-phenylenediamine [24] complexes of
chromium. In all cases, these homoleptic tris-ligand complexes
have been characterized as chromium(III) species with variations
to the oxidation states of the redox-active ligands.
The tridentate pincer-type ligand derived from bis(3,5-di-tert-
butyl-2-phenol)amine, [ONOcat]H3, is a well-established redox-
active ligand that is accessible in three ligand oxidation states
when part of a coordination complex, Scheme 1. The ligand plat-
form has been used to prepare a variety of homoleptic transition
metal complexes of the general formula M[ONO]2, yet surprisingly,
2. Materials and methods
2.1. General considerations
Manipulations were performed using standard Schlenk line
techniques or in a N2 filled glovebox. Diethyl ether, pentane, ben-
zene, toluene, and tetrahydrofuran were sparged with argon and
then dried and deoxygenated by passage through activated alu-
mina and Q5 columns. Pyridine was dried over elemental sodium,
⇑
Corresponding author.
E-mail address: aheyduk@uci.edu (A.F. Heyduk).
https://doi.org/10.1016/j.poly.2017.08.043
0277-5387/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: A.M. Hollas et al., Polyhedron (2017), https://doi.org/10.1016/j.poly.2017.08.043

2
A.M. Hollas et al. / Polyhedron xxx (2017) xxx–xxx
Scheme 1.
vacuum transferred, and stored over molecular sieves. The ligand
precursors [ONOcat]H3 and [ONOq]K were prepared according to
published procedures [26,27].
2.4.1. [ONOq]CrCl2(THF) (1)
A 20-mL scintillation vial was charged with 106 mg of CrCl3(-
THF)3 (0.283 mmol, 1 equiv) and 3 mL of THF. This mixture was
stirred vigorously while a solution containing 130 mg of [ONOq]K
(0.281 mmol, 0.99 equiv) dissolved in 12 mL of THF was added
drop-wise over the course of 15 min. The resulting mixture was
stirred for 3 h then taken to dryness under reduced pressure. The
residue was extracted with toluene, filtered, and again taken to
dryness under reduced pressure. The crude product was dissolved
in THF and layered with pentane. After mixing of the layers over-
night, the solution was placed in a ꢀ35 °C freezer. Crystalline
material was collected by filtration, dissolved in toluene, filtered,
and dried to yield the product as a dark maroon solid in 41% yield
(0.071 g). Anal. Calc. for C32H47NO3Cl2Cr: C, 62.23; H, 7.68; N, 2.27.
Found: C, 61.92; H, 8.10; N, 1.87. UV–vis-NIR (toluene) kmax/nm
2.2. Physical methods
Solution UV–vis-NIR spectra were recorded in 1-cm path-length
cuvettes on a Shimadzu UV-1700 spectrophotometer. Extinction
coefficients were determined from Beer–Lambert Law plots. EPR
spectra were collected on a Bruker EMX X-band spectrometer
equipped with an ER041XG microwave bridge. Solution magnetic
moments (leff) were determined using Evans Method [28–30].
Electrospray ionization mass spectrometry (ESI-MS) data were col-
lected on a Waters LCT Premier mass spectrometer. Elemental
analyses were collected on a Perkin-Elmer 2400 Series II CHNS/O
analyzer. Quantitative analysis of biphenyl was performed on a
Hewlitt-Packard 6890 chromatograph equipped with a flame
(e
/Mꢀ1 cmꢀ1): 352 (5830), 450 (9110), 542 (4460), 575 (4540),
827 (9880), 927 (12700).
ionization detector and
methylpolysiloxane (0.25
Scientific DB-5).
a
m
30 m ꢁ 0.32 mm (5% phenyl)-
2.4.2. [ONOsqÅ]CrCl(bpy) (2)
A scintillation vial was charged with 300 mg of 2 (0.486 mmol,
1 equiv) dissolved in 10 mL of THF and the solution was frozen in a
liquid-nitrogen cold well. The vial was removed from the cold well
l
coating) capillary column (J&W
2.3. Electrochemical methods
and immediately upon thawing
a THF solution containing
0.486 mmol of K[bpyÅ] (1 equiv, prepared from 72 mg of KC8 and
76 mg of 2,20-bipyridine) was added. The reaction mixture was
allowed to warm to ambient glovebox temperature and stirred
for 3 h. The reaction mixture was then filtered and taken to dryness
under reduced pressure. The solid residue was then dissolved in
THF and layered with pentane resulting in the precipitation of
the product as a dark green solid in 97% yield (314 mg). Anal. Calc.
for C38H48N3O2ClCr: C, 68.50; H, 7.26; N, 6.31. Found: C, 68.16; H,
Electrochemical measurements were recorded with a Gamry
G300 potentiostat using a standard three-electrode configuration
including a 3.0 mm glassy carbon working electrode, a platinum
wire auxiliary electrode, and a silver wire pseudo-reference elec-
trode. All measurements were made on solutions that contained
1 mM analyte and 0.1 M [Bu4N][PF6] in THF at an ambient temper-
ature of ca. 21 °C in a glovebox under a N2 atmosphere. Potentials
are referenced to Cp2Fe+/0 using an internal standard of ferrocene
or decamethylferrocene (E°0 = ꢀ0.440 V vs Cp2Fe+/0 in THF) [31].
7.25; N, 5.97. UV–vis-NIR (toluene) kmax/nm (e
/Mꢀ1 cmꢀ1): 375
(10500), 458 (6750), 506 (3030), 544 (2400), 623 (3550), 1187
(3150).
2.4. Crystallographic methods
2.4.3. [ONOcat]Cr(py)3 (3)
X-ray diffraction data were collected on single crystals mounted
on a glass fiber using a Bruker SMART APEX II diffractometer. Mea-
surements were carried out using Mo Ka (k = 0.71073 Å) radiation,
A 20-mL scintillation vial was charged with 264 mg of CrCl3(-
THF)3 (0.264 g, 0.704 mmol, 1 equiv), 4 mL of pyridine and 2 mL
of THF. This mixture was stirred rapidly and a 5-mL THF solution
containing [ONOcat]Li3 (0.691 mmol, 0.98 equiv, prepared from
[ONOcat]H3 and nBuLi) was added slowly. The reaction mixture
was stirred overnight at ambient glovebox temperature (ca.
21 °C). The resulting mixture was concentrated to 2 mL under
reduced pressure and then diluted with 100 mL of a 1:1 benzene:-
toluene solution and filtered. Pyridine was added to the mother
liquor, which was taken to dryness under reduced pressure. The
resulting solid was washed with pentane to remove a magenta
impurity then dissolved in minimal diethyl ether. The addition of
pentane resulted in the precipitation of the product as a green
solid, leaving behind a brown impurity in the mother liquor. The
wavelength selected with a single-crystal graphite monochroma-
tor. A full sphere of data was collected for each crystal structure.
The APEX [32] program package was used to determine the unit cell
parameters and for data collection. The raw frame data was pro-
cessed using SAINT [33] and SADABS [34] to yield the reflection data
files. Subsequent calculations were carried out using the SHELXTL
[35] program. The structures were solved by direct methods and
refined on F2 by full matrix least-squares techniques. The analytical
scattering factors [36] for neutral atoms were used throughout the
analysis. Hydrogen atoms were included using a riding model.
ORTEP diagrams were generated using ORTEP-3 for Windows [37].
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A.M. Hollas et al. / Polyhedron xxx (2017) xxx–xxx
3
green product was obtained as green crystalline rods in 44% yield
(217 mg) by the diffusion of acetonitrile into a pyridine solution
of the complex. Anal. Calc. for C43H55N4O2Cr: C, 72.54; H, 7.79; N,
7.87. Found: C, 72.55; H, 7.80; N, 8.16. UV–vis-NIR (toluene)
which [ONOsqÅ]CrCl(bpy) (2) was isolated in nearly quantitative
yield. Complex 2 has a eff of 2.76 B in room temperature solution,
indicative of the presence of two unpaired electrons and consistent
l
l
with antiferromagnetic coupling between an S = 3/2 chromium(III)
kmax/nm (
e
/Mꢀ1 cmꢀ1): 352 (15900), 408 (3820), 730 (1640).
metal center and an S = ½ [ONOsqÅ 2ꢀ
] ligand. Access to a
chromium(III) complex of the fully reduced, catecholate form of
the [ONO] ligand was achieved through the reaction of CrCl3(THF)3
with trianionic [ONOcat]Li3, which afforded [ONOcat]Cr(py)3 (3) in
modest yield. Complex 3 is paramagnetic; however, Evans’ Method
solution magnetic moment measurements for 3 failed due to poor
solubility.
2.4.4. [(ONOcat)Cr(py)2]2 (4)
scintillation vial containing 157 mg of
1 equiv), 300 L of pyridine (3.72 mmol, 15 equiv), and 8 mL of
A
1 (0.254 mmol,
l
THF was placed in a liquid-nitrogen cold well. The vial was
removed and immediately upon thawing, the cold solution was
added to a cold, vigorously stirred suspension of KC8 (0.071 g,
0.526 mmol, 2.1 equiv) in 2 mL of THF. The reaction mixture was
allowed to come to ambient glovebox temperature and was stirred
for an additional 3 h. After filtration and removal of the volatiles
under reduced pressure, the solid residue was dissolved in pyridine
and layered with acetonitrile. Diffusion of the layers together
resulted in the precipitation of the product as green blocks in
28% yield (45 mg). Anal. Calc. for C76H100N6O4Cr2: C, 72.12; H,
7.96; N, 6.64. Found: C, 71.80; H, 7.96; N, 6.92. UV–vis-NIR
Complexes 1–3 were all characterized by single-crystal X-ray
crystallography, providing insight into both their gross structural
features and their ligand oxidation states. Fig. 1 shows ORTEP dia-
grams and Table 1 shows selected metrical parameters for com-
plexes 1–3. The quinonate complex, [ONOq]CrCl2(THF) (1)
crystallized in the monoclinic space group P21/c with one unique
chromium molecule per unit cell. In the case of semiquinonate
complex [ONOsqÅ]CrCl(bpy) (2), which crystallized in the mono-
clinic space group C2/c, the asymmetric unit contained two unique
chromium molecules. Finally, the catecholate complex [ONOcat
]
(toluene) kmax/nm (e
/Mꢀ1 cmꢀ1): 312 (19600), 514 (1170), 610
Cr(py)3 (3) formed an orthorhombic lattice (Pbca) that also con-
tained two unique chromium molecules per asymmetric unit. All
three complexes contain a six-coordinate chromium center with
the [ONO] ligand bound in a meridional fashion. Bond distances
between the chromium and the oxygen and nitrogen donor atoms
of the [ONO] ligand vary significantly across the series of three
complexes. For example, the distance from the chromium to the
[ONO] nitrogen atom decreases from 2.00 Å in 1 to 1.93 Å in 2 to
1.92 Å in 3. Similarly, the average Cr–O distance (for the [ONO]
ligand) decreases from 1.96 Å in 1 to 1.94 Å in 2 and 3. In contrast,
the metal–ligand distances for the ancillary ligands do not change
much at all across the series. The Cr–Cl(1) distances in both 1 and 2
are 2.32 Å; similarly, the Cr–N distances to the bpy and py ligands
of 2 and 3, respectively, are remarkably consistent. The structural
features for 1–3 suggest the presence of a chromium(III) metal cen-
ter across the series with changes to the [ONO] ligand oxidation
state. Consistent with this point, changes in [ONO] intraligand
bond distances across the series follow the expected trend in
ligand oxidation state. Notably, the average C–N distance increases
from 1.36 Å in 1 to 1.38 Å in 2 to 1.39 Å in 3, while the average C–O
distance increases even more dramatically from 1.28 Å in 1 to
1.32 Å in 2 to 1.36 Å in 3, consistent with a progression from
(1050).
2.5. Addition of PhLi to [ONOq]CrCl2
A 20-mL scintillation vial containing 50 mg of 2 (0.081 mmol,
1 equiv) dissolved in 5 mL of THF and a stirbar was placed in a liq-
uid-nitrogen cold well. The vial was removed from the cold well
and immediately upon thawing a solution of PhLi was added
(0.162 mmol, 2 equiv). The reaction mixture was allowed to warm
to ambient glovebox temperature and was stirred for an addition
3 h. An aliquot of the solution was then removed for analysis by
gas chromatography.
2.6. Addition of 4-tBuC6H4Li to [ONOq]CrCl2
A vial was charged with 58 mg of 2 (0.094 mmol, 1 equiv) dis-
solved in 5 mL of THF and a stirbar and then placed in a liquid-
nitrogen cold well. The vial was removed from the cold well and
immediately upon thawing of the solution, 27 mg of 4-tert-
butylphenyllithium (0.197 mmol, 2.1 equiv) dissolved in 3 mL of
cold THF was added. The solution was allowed to warm to ambient
glovebox temperature and was stirred overnight. The reaction mix-
ture was taken to dryness under reduced pressure and the solid
residue was extracted with several small aliquots of pentane. The
pentane fractions were combined, filtered, and taken to dryness
to afford 5 mg of 4,40-di-tert-butylbiphenyl (0.019 mmol, 20%
yield).
[ONOq]1ꢀ in 1 to [ONOsqÅ 2ꢀ in 2 to [ONOcat 3ꢀ
] in 3. These ligand
]
oxidation state assignments are further supported by the calcu-
lated metrical oxidation states (MOS) for the [ONO] ligands [25],
listed in Table 1.
The electronic absorption spectra readily distinguish between
the different ligand oxidation states of complexes 1–3. Whereas
octahedral chromium(III) complexes are expected to display low-
intensity ligand-field transitions through the UV–vis region of the
spectrum, Fig. 2 shows intense absorption bands for complexes
1–3 that extend from the UV to the near-IR regions of the spec-
trum. Notably, [ONOq]CrCl2(THF) (1) shows intense absorption
bands at 827 and 927 nm, consistent with the previously reported
spectra of complexes containing the quinonate form of the [ONO]
ligand [38–42]. For [ONOsqÅ]CrCl(bpy) (2), a diagnostic transition
3. Results and discussion
3.1. Synthesis and characterization of monomeric chromium(III)
complexes
Monomeric chromium(III) complexes of the [ONO] ligand in all
three ligand oxidation states were readily obtained using salt
metathesis reactions as shown in Scheme 2. Previous efforts in
our lab have shown that the ligand precursor [ONOq]K is an effec-
tive and mild transfer reagent for the oxidized form of the [ONO]
ligand [27]; accordingly, the reaction of [ONOq]K and CrCl3(THF)3
proceeded to afford [ONOq]CrCl2(THF) (1) in 41% yield. Complex
at 1187 nm in the near-IR is consistent with an
p ?
p⁄ transition
within the open-shell ligand [26,43–45]. In the case of fully
reduced [ONOcat]Cr(py)3 (3), intense, high-energy absorptions are
observed at 352 and 408 nm, with a broad, lower intensity absorp-
tion at 730 nm. This latter spectrum is consistent with the spectra
reported for [ONOcat]Fe(py)3 and [ONOcat]Rh(py)3 [27,46].
1 has an effective magnetic moment (leff) of 3.55 lB in solution
at 20 °C, which is consistent with an S = 3/2 chromium(III) metal
center. The addition of one equivalent of K[bpyÅ] to a maroon solu-
tion of 1 resulted in the formation of a dark green solution, from
The frontier electronic manifolds of complexes 1–3 were further
probed using solution cyclic voltammetry. Fig. 3 shows the voltam-
mograms recorded on 1 mM solutions of each complex in THF
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4
A.M. Hollas et al. / Polyhedron xxx (2017) xxx–xxx
Scheme 2.
N(2)
Cl(1)
Cl(1)
N(1)
O(1)
N(1)
O(2)
N(1)
O(1)
O(2)
O(3)
O(2)
N(3)
Cr(1)
Cl(2)
Cr(1)
O(1)
Cr(1)
N(4)
N(3)
N(2)
[ONOq]CrCl2(THF)
[ONO ]CrCl(bpy)
[ONOcat]Cr(py)3
(1)
(2)
(3)
Fig. 1. ORTEP diagrams of [ONOq]CrCl2(THF) (1), [ONOsqÅ]CrCl(bpy) (2), and [ONOcat]Cr(py)3 (3). Ellipsoids are shown at 50% probability. Only one crystallographically unique
molecule from the unit cell is shown for 2 and 3. Hydrogen atoms and solvent molecules have been omitted for clarity.
using a glassy-carbon working electrode with 0.1 M [Bu4N][PF6] as
the supporting electrolyte. Key redox potentials, referenced to
[Cp2Fe]+/0 using an internal standard added to the cell at the end
of an experimental run, are collected in Table 2. All three com-
plexes 1–3 show one-electron redox processes associated with
changes to the oxidation state of the [ONO] ligand. For [ONOq]
CrIIICl2(THF) (1), the first reduction, E°0(0/1–) at –0.49 V, corresponds
to the reduction of the ligand from the quinonate to the semiquino-
CrIIICl2(THF)2ꢀ appears as an irreversible signal at –1.53 V. The
redox chemistry of 1 is completed by an oxidation at +0.91 V, tenta-
tively assigned to the CrIV/III redox couple. The first reduction of
[ONOsqÅ]CrIIICl(bpy) (2), E°0(0/1–) at –1.43 V, corresponds to the
reduction of the semiquinonate ligand to the catecholate ligand:
[ONOsqÅ]CrIIICl(bpy)0 ? [ONOcat]CrIIICl(bpy)1ꢀ
;
whereas, the first
oxidation of 2, E°0(1+/0) at –0.10 V, corresponds to the reduction of
quinonate to semiquinonate: [ONOq]CrIIICl(bpy)1+ ? [ONOsqÅ
]
nate form, [ONOq]CrIIICl2(THF)0 ? [ONOsqÅ]CrIIICl2(THF)1ꢀ
second ligand-based reduction, [ONOsqÅ]CrIIICl2(THF)1ꢀ ? [ONOcat
.
The
CrIIICl(bpy)0. At more negative potentials, bpy-based reductions of
]
2 were observed [13]. The [ONO] ligand redox processes are shifted
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A.M. Hollas et al. / Polyhedron xxx (2017) xxx–xxx
5
more anodically in [ONOcat]CrIII(py)3 (3), with the semiquinonate to
catcholate transition observed as the first oxidation, E°0(1+/0) at –
0.97 V, [ONOsqÅ]CrIII(py)31+ ? [ONOcat]CrIII(py)03, and the quinonate to
semiquinonate transition as the second oxidation, E°0(2+/1+) at
+0.32 V, [ONOq]CrIII(py)32+ ? [ONOsqÅ]CrIII(py)13+. In complex 3, a sin-
gle irreversible reduction is observed near the solvent window,
which can be attributed either to a pyridine-based reduction or to
a CrIII/II reduction. Overall, the electrochemical behavior of 3 is strik-
ingly similar to that of its rhodium analogue (ONOcat)Rh(py)3, which
shows reversible oxidations at ꢀ0.91 and 0.12 V and an irreversible
reduction event at ꢀ2.87 V [26].
Table 1
Selected bond distances and angles for [ONOq]CrCl2(THF) (1), [ONOsqÅ]CrCl(bpy) (2),
and [ONOcat]Cr(py)3 (3). For 2 and 3, the listed bond distances are for one of the
unique chromium molecules in the asymmetric unit.
(ONOq)CrCl2(THF)
(ONOsq)CrCl(bpy)
(ONOcat)Cr(py)3
(1)
(2)
(3)
Bond Lengths (Å)
Cr(1)–N(1)
Cr(1)–O(1)
Cr(1)–O(2)
Cr(1)–Cl(1)
Cr(1)–Cl(2)
Cr(1)–O(3)
Cr(1)–N(2)
Cr(1)–N(3)
Cr(1)–N(4)
N(1)–C(6)
N(1)–C(20)
O(1)–C(1)
O(2)–C(15)
Calc. MOSa
1.998(3)
1.962(3)
1.974(3)
2.3188(14)
2.3189(14)
2.003(3)
–
1.9263(15)
1.9413(14)
1.9428(13)
2.3175(6)
–
1.923(2)
1.939(2)
1.939(2)
–
–
–
2.123(3)
2.093(3)
2.130(3)
1.389(4)
1.386(4)
1.356(3)
1.355(3)
ꢀ2.76
–
2.0705(18)
2.1025(16)
–
1.380(2)
1.382(2)
1.314(2)
1.316(2)
ꢀ2.04
3.2. Reduction of [ONOq]CrCl2(THF)
–
–
Chemical reductions of [ONOq]CrCl2(THF) (1) were attempted to
test the interconversion of the [ONO] ligand oxidation states while
coordinated to chromium(III). Given that [ONOcat]Cr(py)3 (3) was
an isolable complex, the chemical two-electron reduction of 1
was pursued in the presence of excess pyridine. The cyclic voltam-
metry data for 1 indicated the need for a relatively strong reducing
agent to achieve the two-electron reduction. The reduction of 1
with two equivalents of potassium graphite in the presence of
excess pyridine resulted in the formation of a dimeric chromium
(III) complex, {[ONOcat]Cr(py)2}2 (4), as a green, crystalline product
in low yields.
1.355(5)
1.362(5)
1.280(5)
1.272(5)
ꢀ1.14
a
See Ref. [25].
The composition and structure of 4 were determined by single-
crystal X-ray diffraction studies. Complex 4 crystallized in the
monoclinic space group P21/c with two unique molecules per
asymmetric unit. Fig. 4 shows an ORTEP diagram of one of the
unique molecules of 4, which adopts an edge-sharing bioctahedral
structure in the solid state, the other molecule is qualitatively sim-
ilar with only minor variations in bond lengths and angles. The
bimetallic core is bridged by the nitrogen donor atoms of the
[ONO] ligands, which traverse the dimer and coordinate one phe-
noxide oxygen atom to each chromium center. The phenoxide oxy-
gen atoms are located trans to one another leaving two cis
coordination sites for the coordinated pyridine molecules. Overall,
the dimer displays nominal C2 symmetry in the solid state. The Cr–
Cr distances are 2.9523(4) and 2.8991(4) Å, for the two unique
molecules. These distances are similar to those reported for other
chromium dimers bridged by azanido ligands and fall just outside
of the range expected for a formal metal–metal bond (the chro-
mium covalent radius is 1.39 Å) [47]. The effective magnetic
Fig. 2. Electronic absorption spectra for [ONOq]CrCl2(THF) (1), [ONOsqÅ]CrCl(bpy)
(2), and [ONOcat]Cr(py)3 (3) collected in toluene at 20 °C.
moment for 4 was determined to be 3.08
lB in solution at 20 °C,
indicating an anti-ferromagnetic interaction between the two
chromium centers. Ligand metrical oxidation state calculations
for 4 indicate that all of the [ONO] ligands are in the reduced, tri-
anionic, [ONOcat 3ꢀ
form.
]
3.3. Arylation reactions of [ONOq]CrCl2(THF)
Arylations of [ONOq]CrCl2(THF)2 (1) were pursued in an attempt
to generate a species capable of undergoing an intramolecular
reductive elimination reaction. Previously we have used the
intramolecular reductive elimination of biphenyl as a test to eval-
Fig. 3. Cyclic voltammograms for [ONOq]CrCl2(THF) (1), [ONOsqÅ]CrCl(bpy) (2), and
[ONOcat]Cr(py)3 (3) dissolved in THF containing 0.1 M [Bu4N][PF6] electrolyte at a
scan rate of 200 mV sꢀ1 using a glassy-carbon working electrode. Potentials are
referenced to [Cp2Fe]+/0
.
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6
A.M. Hollas et al. / Polyhedron xxx (2017) xxx–xxx
Table 2
Electrochemical data for [ONOq]CrCl2(THF) (1), [ONOsqÅ]CrCl(bpy) (2), and [ONOcat]Cr(py)3 (3).
E°0 / V vs [Cp2Fe]+/0
E°0 (1ꢀ/2ꢀ)
ꢀ1.53
ꢀ1.98
ꢀ
E°0 (0/1ꢀ)
ꢀ0.49
ꢀ1.43
ꢀ2.63
E°0 (1+/0)
E°0 (2+/1+)
[ONOq]CrCl2(THF) (1)
[ONOsqÅ]CrCl(bpy) (2)
[ONOcat]Cr(py)3 (3)
+0.91
ꢀ0.10
ꢀ0.97
–
–
+0.32
A preparatory-scale version of the reaction was carried out using
4-tert-butylphenyl lithium, resulting in the isolation of 4,40-di-
tert-butylbiphenyl in 20% yield, consistent with the phenyl lithium
results. In both the phenyl lithium and 4-tert-butylphenyl lithium
reactions the quantitative yields of the coupled products were
lower than those reported for the treatment of CrCl3 with PhLi
[48]. Furthermore, the observed formation of the H-atom abstrac-
tion product benzene in the reaction of PhLi with 1 suggests that a
one-electron pathway is competitive with any reductive elimina-
tion pathway that might be facilitated by the redox-active [ONO]
ligand.
O(2)
O(3)
N(6)
N(2)
N(4)
Cr(1)
Cr(2)
N(3)
N(5)
O(4)
N(1)
4. Summary and conclusions
O(1)
Controlled addition of the redox-active [ONO] pincer ligand
allowed the preparation of monometallic chromium(III) complexes
with all three oxidation state of the ligand. Salts of both the quino-
nate and catecholate forms of the ligand provided access to [ONOq]
CrCl2(THF) (1) and [ONOcat]Cr(py)3 (3), respectively; whereas, the
chromium(III) semiquinonate complex, [ONOsqÅ]CrCl(bpy) (2) was
accessed through the reaction of 1 with the potassium salt of the
bpy radical anion. Surprisingly the two-electron reduction of 1 in
the presence of excess pyridine did not afford 3 as expected, but
rather resulted in the formation of a dimeric species, {[ONOcat]Cr
(py)2}2 (4), in which nitrogen atoms of the [ONO] ligands bridge
the metal centers, effectively preventing the coordination of a third
pyridine ligand. While 4 contains chromium(III) centers and fully
Fig. 4. ORTEP diagram of {[ONOcat]Cr(py)2}2 (4). Only one crystallographically
unique molecule from the unit cell is shown. Ellipsoids are shown at 50%
probability. Hydrogen atoms and solvent molecules have been omitted for clarity.
uate the ability of a redox-active ligand complex to mediate two-
electron small molecule reactions. Further, it is established in the
literature that triphenyl complexes of chromium(III) will eliminate
biphenyl in a bimolecular reaction to produce chromium(II) prod-
ucts [48,49]. We reasoned that the arylation of 1 would produce a
diphenyl chromium complex that could subsequently lose biphe-
nyl in a unimolecular, two-electron reductive elimination reaction.
Ideally, the chromium product of the reaction would be expected
to be a reduced [ONOcat]CrL3 species akin to 3; however, a bimetal-
lic {[ONOcat]CrL2}2 species akin to 4 could also be possible.
reduced [ONOcat 3ꢀ
ligands, the isolation of this product suggests
]
that the mechanism of reduction may involve other metal oxida-
tion states in order to achieve the ligand redistribution required
to access the dimer product. Similarly, the attempted arylation
reactions of 1 suggest that one-electron pathways involving metal
reduction are competitive with more desirable two-electron reduc-
tive elimination pathways. Future work will be directed towards
forcing the [ONO] chromium platform to react in two-electron
steps to effect atom and group transfer reactivity.
Acknowledgements
We thank Dr. Jordan Corbey for assistance with the collection
and analysis of single-crystal X-ray diffraction data and the
National Science Foundation (CHE-1464832) for financial support.
Appendix A
CCDC 1565054–1565057 contains the supplementary crystallo-
graphic data for complexes 1–4. These data can be obtained free of
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
The arylation of quinonate complex 1 resulted in the formation
of the homocoupled bis(aryl) product in substoichiometric yields.
In a typical reaction, a freshly-thawed solution of 1 in THF was
treated with two equivalents of PhLi and allowed to warm to ambi-
ent temperature. The reaction progress was evident from the loss
of the dark maroon color associated with 1, and the UV–vis-NIR
spectrum of the final solution showed only the broad features in
the UV–vis region that are associated with the [ONOcat 3ꢀ
]
form of
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