Photolytic reactivity of organometallic chromium bipyridine complexes

Article abstract of DOI:10.1021/acs.inorgchem.7b03195

Known stable [Cr(bpy)₂(Ph)₂](BPh₄) complexes undergo reductive elimination of biphenyl with visible-light photolysis using household incandescent or compact fluorescent light bulbs. A series of [Cr(R-bpy)₂(Ar)₂](X) complexes (R = H or CMe₃; Ar = Ph, C₆H₄-CMe₃, or C₆H₄-OMe; X = I, BPh₄, or PF₆) were prepared, and the effect of varying the bipyridine and aryl ligands on the UV-visible spectra and electrochemistry of the chromium(III) complexes was investigated. Photolysis of a mixture of two different bis(aryl) complexes gave only the homocoupled biaryl products by¹H NMR and gas chromatography/mass spectrometry analysis. The initial product of photoinduced reductive elimination of [Cr(bpy)₂(Ar)₂](PF₆) was trapped with bipyridine to generate [Cr(bpy)₃](PF₆) and with benzoyl peroxide to form [Cr(bpy)₂(O₂CPh)₂](PF₆). The latter chromium(III) bis(benzoate) complex was also synthesized by the addition of bipyridine and PhCO₂H to Cp₂Cr, followed by air oxidation. The neutral Cr(bpy)(S₂CNMe₂)Ph₂complex also generated biphenyl upon visible-light photolysis. While the treatment of Cr(^tBu-bpy)(dpm)Cl₂[dpm = (OC^tBu)₂CH] with AgO₂CPh gave trans-Cr(^tBu-bpy)(dpm)(O₂CPh)₂, reaction of the dichloro precursor with PhMgCl produced anionic [Cr(^tBu-bpy)Ph₃]^?with [Mg(dpm)(THF)₄]⁺as the countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph₃(THF) with 8-hydroxyquinoline produced Cr(bpy)(quin)Ph₂, which generated biphenyl under visible-light photolysis, and the initial product of reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr(bpy)(quin)₂complex was synthesized by protonolysis of Cr(bpy)[N(SiMe₃)₂]₂and characterized by single-crystal X-ray diffraction.

Full text of DOI:10.1021/acs.inorgchem.7b03195

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Photolytic Reactivity of Organometallic Chromium Bipyridine
Complexes
†
†
†
‡
Benjamin E. Olafsen, Giuseppe V. Crescenzo, Luke P. Moisey, Brian O. Patrick,
,
†
and Kevin M. Smith*
†Department of Chemistry, University of British Columbia, Okanagan, 3247 University Way, Kelowna, British Columbia V1V 1V7,
Canada
‡
Department of Chemistry, University of British Columbia, Vancouver, Vancouver, British Columbia V6T 1Z21, Canada
*
S Supporting Information
ABSTRACT: Known stable [Cr(bpy) (Ph) ](BPh ) complexes undergo reductive
2
2
4
elimination of biphenyl with visible-light photolysis using household incandescent or
compact fluorescent light bulbs. A series of [Cr(R-bpy) (Ar) ](X) complexes (R = H or
2
2
CMe ; Ar = Ph, C H -CMe , or C H -OMe; X = I, BPh , or PF ) were prepared, and the
3
6
4
3
6
4
4
6
effect of varying the bipyridine and aryl ligands on the UV−visible spectra and
electrochemistry of the chromium(III) complexes was investigated. Photolysis of a mixture
1
of two different bis(aryl) complexes gave only the homocoupled biaryl products by H
NMR and gas chromatography/mass spectrometry analysis. The initial product of
photoinduced reductive elimination of [Cr(bpy) (Ar) ](PF ) was trapped with bipyridine
2
2
6
to generate [Cr(bpy) ](PF ) and with benzoyl peroxide to form [Cr(bpy) (O CPh) ]-
3
6
2
2
2
(
PF ). The latter chromium(III) bis(benzoate) complex was also synthesized by the
6
addition of bipyridine and PhCO H to Cp Cr, followed by air oxidation. The neutral
2
2
Cr(bpy)(S CNMe )Ph complex also generated biphenyl upon visible-light photolysis.
2
2
2
t
t
t
While the treatment of Cr( Bu-bpy)(dpm)Cl [dpm = (OC Bu) CH] with AgO CPh gave trans-Cr( Bu-bpy)(dpm)(O CPh) ,
2
2
2
2
2
t
−
+
reaction of the dichloro precursor with PhMgCl produced anionic [Cr( Bu-bpy)Ph ] with [Mg(dpm)(THF) ] as the
3
4
countercation, with both complexes characterized by single-crystal X-ray diffraction. Protonolysis of Cr(bpy)Ph (THF) with 8-
3
hydroxyquinoline produced Cr(bpy)(quin)Ph , which generated biphenyl under visible-light photolysis, and the initial product of
2
reductive elimination was trapped by bipyridine or benzoyl peroxide. A related Cr(bpy)(quin) complex was synthesized by
2
protonolysis of Cr(bpy)[N(SiMe ) ] and characterized by single-crystal X-ray diffraction.
3
2 2
INTRODUCTION
drofuran (THF) ligand dissociation preceded reductive
elimination to form chromium(I) η -arene complexes. By
■
6
Open-shell organometallic complexes of first-row transition
metals have experienced increasing use in catalytic C−C bond-
using bpy to prevent ligand dissociation, Zeiss and co-workers
prepared unusually stable cationic chromium(III) bis(phenyl)
1
forming reactions. Catalysis research involving photolysis,
11
compounds. As shown in Scheme 1, neutral complex 1 with
radicals, and redox-active ligands has been a particularly active
2
two unpaired electrons was initially generated from CrCl , with
area. In these reactions, mechanistic studies using well-defined
2
complexes have provided the critical insights required to design
3
improved catalysts. For several first-row metals, bipyridine
Scheme 1. Zeiss Synthesis of Air- and Water-Stable
Chromium(III) Bis(aryl) Complexes
4
(
bpy) ligands have been shown to be especially effective. The
connection between the ancillary ligand redox activity and
5
catalyst performance remains the focus of ongoing research.
Recently, the groups of Knochel and Zeng have reported
2
cross-coupling reactions of arylmagnesium halides and C(sp )−
X bonds (X = OMe, Cl, H) catalyzed by chromium dichloride
6
(
CrCl2). While chromium-catalyzed C−C bond-forming
7
reactions are underdeveloped relative to iron, cobalt, or nickel,
the stoichiometric reactions of chromium and aryl Grignard
reagents have a long and rich history. The homocoupling of
8
PhMgBr to biphenyl using chromium trichloride (CrCl ) was
3
Special Issue: Applications of Metal Complexes with Ligand-
Centered Radicals
9
first reported over a century ago. Although Hein isolated
8
unusual organochromium complexes from these reactions,
their nature remained poorly understood until Zeiss and co-
Received: December 19, 2017
10
workers isolated CrPh (THF) , and showed that tetrahy-
3
3
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03195
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
II
3+
Scheme 2. (a) Electron Transfer from Cr to bpy and (b) Sequential Reduction of bpy Ligands in [Cr(bpy) ]
3
Table 1. UV−Visible Absorbance and Electrochemical Data for [Cr(R-bpy) (C H -R) ][BPh ]
2
6
4
2
4
a
ε /M cm−1
a
−1
E1/2+/0 /V
b
E1/20/− /V
b
complex
R-bpy
C H -R
λmax /nm
6
4
1
(BPh4)
a(BPh4)
(BPh4)
a(BPh4)
bpy
C H
373
369
385
382
2750
2970
3060
3570
−1.56
−1.69
−1.60
−1.71
−2.07
−2.24
−2.14
−2.30
6
5
5
t
1
2
2
Bu-bpy
C H
6
t
bpy
C H - Bu
6 4
t
t
Bu-bpy
C H - Bu
6 4
a
b
+/0
In acetonitrile. Potentials reported versus [Cp Fe] . In THF at room temperature with 0.1 M [NBu ][PF ].
2
4
6
subsequent air oxidation in aqueous methanol containing
potassium iodide, resulting in an octahedral chromium(III)
cationic complex 1(I) with three unpaired electrons.
SYNTHESIS OF CATIONIC CHROMIUM(III)
BIS(ARYL) COMPLEXES
■
1
1d,e
Complex 1(I) was prepared following the procedure of
11e
Octahedral S = 1 bpy complexes related to 1 have been
Sneeden and Zeiss (Scheme 1).
The use of NaBPh or
4
shown to possess anionic bpy-ligand-based radicals antiferro-
KPF instead of KI during the air-oxidation step resulted in the
6
III
3
II
4
magnetically coupled to Cr d rather than the low-spin Cr d
corresponding 1(BPh ) or 1(PF ) salts, respectively. Replacing
4
6
12
t
structure they were originally assigned. Such complexes can
PhMgCl with the corresponding 4- Bu-C H or 4-OMe-C H
reagents generated [Cr(bpy) (C H -R) ] (2 , R = Bu; 3 R =
OMe). For the C H and C H - Bu complexes, corresponding
6 4 6 4
+
+
t
+
be prepared directly from well-defined chromium(II) com-
2
6
4
2
t
1
3
plexes, such as Theopold and Wieghardt’s recent reinvesti-
gation of Herzog’s reaction of an anhydrous chromium(II)
6
5
6
4
t
cationic compounds with 4,4′-di-tert-butylbipyridine ( Bu-bpy)
+
+
14
were also prepared, 1a and 2a , respectively.
acetate dimer and bpy (Scheme 2A). Wieghardt has also
The effects of introducing tert-butyl substituents on the
electrochemistry and UV−visible spectra of the parent cationic
3
+
demonstrated that the reduction of [Cr(bpy)3] leads to
sequential occupation of the bpy π-antibonding orbitals, while
+
t
complex 1 are shown in Table 1. The Cr-C H - Bu complexes
6
4
III
the central metal remains Cr (Scheme 2B; reduction
have λmax values at lower energy compared to the
+
/0 15
potentials referenced to [Cp Fe] ).
The reduced
2
corresponding Cr-C H compounds. Replacing the bpy ligands
6
5
t
chromium(III) compounds with bpy-ligand-based radicals
with 4,4′- Bu-bpy results in a smaller shift in λ to higher
energy. In cyclic voltammetry, reversible single-electron
reductions for all of the bis(aryl) compounds are observed at
max
typically have strong absorbances in the 500−700 nm region
13
of their UV−visible spectra. The inter-ring C(py)−C(py)
bond length determined by single-crystal X-ray diffraction has
been used to assign the oxidation level of bpy ligands in
3
+
more negative potential than the corresponding [Cr(bpy) ]
3
20
values shown in Scheme 2B. This is consistent with both the
decreased positive total charge on the monocationic organo-
metallic complexes and the presence of two strongly σ-donating
16
reduced chromium complexes.
Studies of well-defined organochromium compounds have
anionic phenyl ligands. Replacing the phenyl ligands with
17
led to improved C−C bond-forming catalysts, especially for
t
C H - Bu leads to a slight shift to even more negative reduction
18
6
4
ethylene polymerization as well as ethylene trimerization and
potentials. A larger negative reduction potential change is
1
9
tetramerization. The unusual stability of the chromium(III)
bis(aryl) complexes in Scheme 1 made them attractive as bpy-
t
observed when bpy is replaced with Bu-bpy. This larger effect
is consistent with the proposed electronic structures of the
stabilized models for potential intermediates in CrCl -catalyzed
reduced species, where the added electrons singly occupy the π-
antibonding orbitals of the Bu-bpy ligands.
2
t
reactions involving aryl Grignard reagents. In this paper, we
report our investigation of known and new chromium
bipyridine bis(phenyl) complexes, including their unexpected
reductive elimination induced by visible light. Understanding
the mechanism of the reactivity of metal complexes with ligand-
centered radicals is a necessary step for the rational design of
ancillary ligands for catalytic C−C bond formation.
PHOTOLYTIC REACTIVITY
■
The photochemistry of octahedral chromium(III) compounds
21
has been extensively explored since the 1960s. Recent
photochemical research on [Cr(bpy)3]3+ and related phenan-
throline and terpyridine derivatives have exploited the reactivity
B
DOI: 10.1021/acs.inorgchem.7b03195
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Inorganic Chemistry
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−4
Figure 1. UV−visible spectra of [Cr(bpy) Ph ][BPh ] (3.3 × 10 M in THF) before photolysis (red), after 1 h of photolysis (purple), and after air
2
2
4
exposure of a photolyzed solution (yellow).
Scheme 3. Proposed Mechanism of the Photoinduced Reductive Elimination of Biphenyl
differences of these first-row transition-metal complexes
of additional bpy, the growth was slower and showed an initial
compared to the widely used ruthenium(II) and iridium(III)
induction period, consistent with the initial decomposition of
2
2
+
+
catalysts. Although the [Cr(bpy) Ar ] complexes originally
the [Cr(bpy) ] photoproduct being required to generate free
2
2
2
prepared by Mu
̈
ller and Zeiss are remarkably stable with respect
bpy in solution. Finally, no strong absorbance in the 500−700
11
to air, water, and reductive elimination, the stabilizing bpy
ligands also open up a new, previously unrecognized C−C
bond-forming mechanism, via visible-light photolysis. As shown
in Figure 1, a 3.3 × 10 M THF solution of 1(BPh ) under
nitrogen turns dark purple when photolyzed with a household
nm region was observed during the photolysis of [Cr-
+
(bpy) Ph ] in the presence of excess ZnCl , presumably
2
2
2
because of favorable Zn(bpy)Cl formation rendering bpy
2
−4
unavailable for chromium binding (see the Supporting
Information). Note that these UV−visible monitoring results
are not consistent with the standard mechanism for reductive
elimination from octahedral chromium(III) bis(aryl) com-
plexes, which requires the initial formation of a coordinatively
4
23 W compact fluorescent light bulb at 10 cm for 1 h. The
distinctive bands in the 500−700 nm region characteristic of a
bpy-based radical disappear when the photolyzed solution is
exposed to air. The air-exposed solution also has a marked
decrease in absorbance at ∼375 nm, the λ
chromium(III) bis(phenyl) starting material. Exhaustive visible-
light photolysis of 2(I) with 1,3,5-C H (OMe) as an internal
standard generated 94% 4,4′-bis(tert-butyl)biphenyl by
NMR.
8
4
,19a−f
unsaturated intermediate2,
e.g., by photoinduced dissoci-
value of the
ation of the bpy ligand or by ZnCl -mediated removal of
max
2
25
bpy.
Photolysis of [Cr(bpy) (C H Bu) ][BPh ] was performed
t
4
6
3
3
2
6
2
4
1
H
using various long-pass filters to establish which wavelength of
light was responsible for inducing reductive elimination. As
shown in Figure 2, the use of a filter with a cut-on wavelength
of 475 nm, indicated by the vertical yellow line, resulted in no
change in the UV−visible spectra after 85 min of photolysis. In
The proposed mechanism for the photolysis process is
shown in Scheme 3. The presumed initial product of the
photoinduced reductive elimination of biphenyl would be
+
[
Cr(bpy) ] , which is expected to decompose in the absence of
contrast, 60 min of photolysis with 435 nm (blue) or 395 nm
2
+
trapping reagents. The UV−visible spectra of the photolyzed
solutions matches that of [Cr(bpy) ] (Scheme 2b). Support
for this mechanism was obtained by monitoring the UV−visible
spectra of [Cr(bpy) Ph ] during photolysis. The rate of
growth of the strong visible absorbances was markedly
increased in the presence of excess added bpy. In the absence
(red) filters resulted in the spectra of [Cr(bpy) ] , with
3
+
23
increased intensity observed for the 395 nm long-pass filter.
Further mechanistic information was provided by the
crossover experiment shown in Scheme 4. After a 1:1 mixture
of 2(I) and 3(I) was photolyzed overnight in THF with 1,3,5-
3
+
2
2
1
tris(isopropyl)benzene as the internal standard, H NMR
C
DOI: 10.1021/acs.inorgchem.7b03195
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Inorganic Chemistry
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t
−4
Figure 2. UV−visible of [Cr(bpy) (C H Bu) ][BPh ] (3.3 × 10 M in THF) after photolysis with long-pass filters with cut-on wavelengths of 395
2
6
4
2
4
nm (red), 435 nm (blue), and 475 nm (yellow).
Scheme 4. Crossover Photolysis Experiment
Scheme 5. Trapping the Chromium Product of Reductive Elimination with Benzoyl Peroxide
analysis of the products after workup showed >95% yield of the
two homocoupled biaryl products only. The identity of the
products was confirmed by repeating the experiment and
analyzing the products by gas chromatography/mass spectrom-
etry (GC/MS). The absence of a mixed biaryl product is
consistent with the concerted reductive elimination from the
initial photoexcited complex. The absence of tert-butylben-
zene and anisole from both experiments is notably inconsistent
with the generation of aryl radicals, which would be expected to
rapidly abstract a H atom from the THF solvent.
As shown in Scheme 5, benzoyl peroxide was used to trap the
+
[Cr(bpy)
While no reaction between [Cr(bpy) Ph ](PF ) and (PhCO )
2 2
] product of photoinduced reductive elimination.
2
2
2
6
was observed in the dark by UV−visible, photolysis of 1(PF )
6
and benzoyl peroxide resulted in a red solution of [Cr-
(
(
bpy) (O CPh) ](PF ). If the electronic structure of [Cr-
2 2 2 6
+
II
bpy) ] is presumed to be high-spin Cr antiferromagnetically
26
2
16b
coupled to a ligand-based bpy radical anion,
oxidative
addition of the weak O−O bond of benzoyl peroxide would
involve single electrons from both the Cr center and ligand-
27
based radical. This two-electron chemistry is in contrast to
D
DOI: 10.1021/acs.inorgchem.7b03195
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Inorganic Chemistry
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t
the single-electron oxidative addition typically mediated by the
Cr /Cr redox couple.
Scheme 8. Reactions of Cr( Bu-bpy)(dpm)Cl
2
II
III
The chromium(III) bis(benzoate) complex 4(PF ) was
6
independently synthesized, as shown in Scheme 6. Following
Scheme 6. Independent Synthesis of Chromium(III)
Bis(benzoate)
the analogous synthesis of bis(acetate) from chromium(II)
14
shown in Scheme 2a, 4(PF ) was prepared by the addition of
6
bpy and benzoic acid at room temperature, followed by air
oxidation with NH PF in aqueous methanol. Cotton and co-
4
6
workers have previously demonstrated the protonolysis of
28
chromocene with benzoic acid and substituted derivatives. In
this case, protonolysis of the Cr−C H bond may be
5
5
5
1
accelerated by the initial formation of Cr(bpy)(η -Cp)(η -
1
3
Cp).
NEUTRAL BIPYRIDINE BIS(ARYL) COMPLEXES
■
In 1977, West reported that the neutral dithiocarbamate
bipyridine complex was also anomalously stable with respect to
air and water, unlike the analogous complexes with two
29
monodentate pyridine ligands. As shown in Scheme 7, the
Scheme 7. West Synthesis of a Neutral Chromium(III)
Bis(aryl) Complex
Figure 3. Thermal ellipsoid diagram (50%) of 5. All H atoms have
been removed for clarity.
While the donor atoms formed a fairly regular octahedral
coordination environment at chromium(III), distortion of the
dpm ligand out of the coordination plane was attributed to
intermolecular interactions in the unit cell. The inter-ring
C(py)−C(py) distance of 1.495(3) Å in 5 is consistent with a
16
sequential treatment of CrCl with bpy and tetramethylthiuram
neutral bpy ligand.
2
disulfide produces the olive-green CrCl , which can be
However, the reaction of the dichloride with PhMgCl did not
give the expected neutral bis(aryl) complex (Scheme 8). The X-
ray crystal structure of the product 6 is shown in Figure 4. The
anionic chromium tris(phenyl) complex was square-pyramidal,
and while it retained the Bu-bpy ligand, the dpm ligand was
incorporated in the Mg(dpm)(THF)4 countercation. Like
2
converted to the red bis(phenyl) complex with PhMgBr.
Photolysis of the neutral dithiocarbamate bis(phenyl) complex
does generate biphenyl, although the observed yield was lower
than that for the cationic bis(bipyridine) complexes. On the
basis of this promising initial result, alternative bidentate,
monoanionic ligands were investigated to replace the
dithiocarbamate ligand, which is not readily modified near the
metal center and generally leads to complexes with low
solubility.
t
related neutral and anionic five-coordinate chromium(III)
19e
tris(phenyl) complexes, 6 is evidently stable with respect
to the reductive elimination of biphenyl under ambient
conditions despite being coordinatively unsaturated. While β-
In order to obtain a neutral dichloride precursor with
diketonate substitution from Cr(acac) has occasionally been
used to prepare chromium(III) organometallic species, this
was not the expected product. The measured magnetic moment
3
30
31
improved solubility, the known dipivaloylmethane (dpm)
t
complex Cr(dpm)Cl (THF) was reacted with Bu-bpy. As
2
2
shown in Scheme 8, the dichloride was treated with AgO CPh
of 2.95(9) μ corresponds to two unpaired electrons, which is
2
B
II
4
III
3
to give the bis(benzoate) complex 5. In contrast, West’s
dithiocarbamate dichloride did not react cleanly with silver
benzoate under the same conditions, possible because of
competing abstraction of the soft S CNMe ligand with the
consistent with either a low-spin Cr d center or Cr
d
t
antiferromagnetically coupled to a ligand-based Bu-bpy radical
t
anion. The inter-ring C(py)−C(py) distances in the Bu-bpy
ligands of the two independent Cr anions in the unit cell are
1.427(2) and 1.428(2) Å, and the Cr−N bond lengths are
relatively short [between 2.078(2) and 2.086(2) Å], consistent
2
2
silver reagent. Single-crystal X-ray diffraction of 5 showed that
the complex had trans-disposed benzoate ligands (Figure 3).
E
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(
(
quin) gives the neutral bis(aryl) complex Cr(bpy)(quin)Ph2
7).
1
Photolysis of 7 produces biphenyl, as detected by H NMR
(
Scheme 10). Photolysis of 7 in the presence of benzoyl
Scheme 10. Synthetic Routes to 8
Figure 4. Thermal ellipsoid diagram (50%) of one of the two
crystallographically independent cation and anion pairs of 6. All H
atoms have been removed for clarity.
with a bpy radical anion bound to CrIII.16 Also as expected for a
square-pyramidal chromium(III) complex, the Cr−C(ispo)
bond lengths for the axial phenyl ligands are shorter [2.037(2)
and 2.044(2) Å] than those for the phenyl ligands that are trans
to the bpy N-donor atoms, which are 2.085(2), 2.089(2),
peroxide results in Cr(bpy)(quin)(O CPh) (8), which can be
independently synthesized from Cr(bpy)(quin)Cl2 and
2
2
AgO CPh. The use of bpy to trap the initial product of
2
biphenyl reductive elimination gives a UV−visible spectrum
characteristic of a reduced bpy ligand radical complex,
2
.087(2), and 2.100(2) Å.
Gibson and co-workers used protonolysis reactions of
presumably Cr(bpy) (quin). The spectrum is distinct from
2
Cr(C H Me)Cl (THF) with salicylaldimines in toluene to
6
4
2
3
that of the related reduced product, Cr(bpy)(quin) (9), which
2
generate chromium(III) precatalysts for screening ligand effects
can be prepared by protonolysis of Cr(bpy)[N(SiMe ) ] with
32
3 2 2
in ethylene polymerization reactions. Previously synthesized
by the reactions of thermally sensitive CrPh (THF) with
quin (Scheme 11) and has been characterized by single-crystal
X-ray diffraction (Figure 5).
3
3
1
1a
bpy, Cr(bpy)Ph (THF) was an attractive potential proto-
3
nolysis precursor to neutral chromium(III) bipyridine bis(aryl)
33
Scheme 11. Independent Synthesis of 9 via Cr-N(SiMe
Protonolysis
)
3 2
complexes. As shown in Scheme 9, the reaction of known
Cr(bpy)Cl (THF) with PhMgCl led to precipitation of the red
3
18c
Cr(bpy)Ph (THF) product from a THF solution.
The
3
treatment of Cr(bpy)Ph (THF) with 8-hydroxyquinoline
3
Scheme 9. Synthesis of 7 via Cr−Ph Protonolysis
CONCLUSIONS AND OUTLOOK
■
In this paper, we have explored the unexpected visible-light
photolytic reactivity of a class of organochromium complexes
first reported almost half a century ago. The bpy ligand
responsible for the remarkable air and water stability of these
compounds also opens a new route for C−C bond formation.
Both λ and E can be tuned by modifying the substituents
max
1/2
of either the bpy or aryl ligands. The reactivity of the quin
complex may potentially be extended to 8-aminoquinoline
derivatives, which are active substrates in catalytic C−H
functionalization reactions involving cobalt(III) carboxylate
3
4
intermediates. The low-valent chromium intermediates
generated by photoinduced reductive elimination from
chromium(III) bipyridine bis(aryl) complexes are expected to
F
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microanalytical services at the University of British Columbia’s
Vancouver campus. Satisfactory elemental analysis for several of the
complexes could not be obtained, particularly 1(PF ) and 7, which had
6
low analyses for C. The UV−visible spectra for all compounds are
provided in the Supporting Information.
Electrochemical Methods. Cyclic voltammetry was carried out
−
1
under nitrogen at a 100 mV s standard scan rate in THF/0.1 M
NBu ][PF ] using a three-electrode configuration (a glassy carbon
[
4
6
working electrode, a platinum wire counter electrode, and a Ag/AgCl
reference electrode) and a PARSTAT 2273 potentiostat and function
generator. The ferrocene/ferrocenium couple served as an internal
reference. UV−visible spectroelectrochemical measurements were
performed in a 1.0-mm-path-length thin-layer quartz glass spectro-
chemical cell from BAS (012904 SEC-C), using a platinum mesh
working electrode with a throughput transmission of 0.4, a platinum
wire counter electrode, and a Ag/AgCl reference electrode.
Figure 5. Thermal ellipsoid diagram (50%) of 9. All H atoms have
been removed for clarity.
Crystallographic Methods. Single crystals were mounted on a
glass fibers, and measurements were made on a Bruker X8 APEX II
diffractometer with graphite-monochromated Mo Kα radiation. The
data were collected at a temperature of −100 ± 1 °C in a series of φ
and ω scans in 0.50° oscillations. Data were collected and integrated
demonstrate unusual reactivity modes, potentially including
6
activation of C−O, C−Cl, or C−H bonds and reductive
3
9
19,35
using the Bruker SAINT software package and corrected for
40
coupling of unsaturated substrates.
Through an under-
absorption effects using the multiscan technique (SADABS). The
data were corrected for Lorentz and polarization effects, and the
standing of the fundamental organometallic reactions of these
paramagnetic first-row transition-metal complexes, new chro-
41
structure was solved by direct methods. Compound 6 crystallized
2
2
mium-catalyzed C(sp )−C(sp ) bond-forming reactions can be
with two independent pairs of ions in the asymmetric unit. Three of
the tert-butyl substituents on the bpy and diketonate ligands in 6 were
disordered and subsequently modeled in two orientations with the
occupancies of the disorder about C37 (91:19), C74 (85:15), and
C100 (76:24). The cation of Mg1 was completely disordered with O1
and O2 of the dpm ligand in constant positions. The four THF
molecules bound to Mg1 and Mg1 were split into two sets and
modeled separately with an approximate 66:34 occupancy of the two
orientations. Bound to Mg2, one C on a THF ligand was modeled as
C124 and C130 in two positions with an approximate 83:17
occupancy. Compound 9 crystallized with two independent molecules
in the asymmetric unit. Additionally, two disordered molecules of
toluene were also found in each asymmetric unit of 9 and each
modeled in two orientations using rigid groups. All non-H atoms were
refined anisotropically. All refinements were performed using the
explored.
EXPERIMENTAL SECTION
General Synthetic Methods. All reactions were carried out under
nitrogen using standard Schlenk and glovebox techniques unless
■
otherwise indicated. Hexanes, toluene, diethyl ether (Et O),
2
tetrahydrofuran (THF), and dichloromethane (CH Cl ) were purified
2
2
by passage through activated alumina and deoxygenizer columns from
Glass Contour Co. (Laguna Beach, CA). Celite was dried overnight at
1
20 °C before being evacuated and then stored under nitrogen.
Chromium dichloride (CrCl ), chromium trichloride (CrCl ; anhy-
2
3
drous), acetonitrile (anhydrous), 1,4-dioxane (anhydrous), 2,2′-
bipyridine, 4,4′-di-tert-butyl-2,2′-bipyridine, PhMgCl (2.0 M in
THF), 4-MeOC H MgBr (0.5 M in THF), 4-Me CC H MgBr (0.5
6
4
3
6
4
42
M in THF), anhydrous ZnCl2 (0.5 M in THF), potassium
hexafluorophosphate, potassium iodide, sodium tetraphenylborate,
benzoic acid, silver benzoate, 2,2,6,6-tetramethyl-3,5-heptanedione,
and 8-hydroxyquinoline (quin) were purchased from Aldrich and used
SHELXTL crystallographic software package of Bruker AXS. The
43
molecular drawings were generated by using ORTEP-3 and POV-
Ray. Diffraction data are shown in Table 2.
Synthesis of [Cr(bpy)
of bpy (4.04 mmol) in 5 mL of THF was added to a gray suspension
of 254 mg of CrCl (2.07 mmol) in 10 mL of THF, resulting in a dark-
(Ph) ](PF ) [1(PF )]. A solution of 631 mg
2
2 6 6
36
13
37
as received. CrCl (THF) , Cr(bpy)[N(SiMe ) ] , and Cp Cr
3
3
3
2
2
2
were prepared according to literature procedures. Authentic samples of
2
4
,4′-di-tert-butylbiphenyl and 4,4′-dimethoxybiphenyl for a GC/MS
blue suspension. The dropwise addition of 2.2 mL of PhMgCl (2.0 M
in THF, 4.40 mmol) resulted in a red-purple mixture. After stirring for
17 h, the solvent was removed from the deep-violet solution in vacuo.
comparison were prepared by catalytic oxidative homocoupling of the
corresponding ArMgBr reagents with FeCl and 1,2-dichlorobutane
according to the literature procedure. [Cr(bpy) (Ph) ](I) [1(I)] was
3
38
A solution of 1.077 g of KPF (5.85 mmol) in 40 mL of 2:1 methanol/
2
2
6
prepared by the reaction of CrCl with bipyridine (bpy) and PhMgCl
water was added to the purple residue, and air was bubbled through
the stirring suspension for 30 min. The orange precipitate was isolated
2
in THF under nitrogen, followed by air oxidation in an aqueous
1
1e
methanol solution of KI, according to the literature procedure. The
other [Cr(R-bpy) (C H R) ](X) complexes were synthesized by the
on a sintered glass filter and washed with 30 mL of 2:1 MeOH/H
two aliquots. After drying under vacuum, 1.231 g of [Cr(bpy)
(PF ) (92% yield) was isolated as a yellow powder. Anal. Calcd for
P: C, 57.92; H, 3.95; N, 8.44. Found: C, 52.84; H, 4.00;
2
O in
Ph ]-
2
2
6
4
2
2
t
same procedure using bpy or 4,4′- Bu -bpy as a neutral ligand,
6
2
t
C H MgCl, 4-MeO-C H MgCl, or 4- Bu-C H MgCl as an aryl
C H26CrF N
32 6 4
6
5
6
4
6
4
−1
−1
Grignard reagant, and KI, NaBPh , or KPF as a counteranion source.
N, 7.55. UV−visible [NCMe; λmax, nm (ε, M cm )]: 373 (2300).
Synthesis of [Cr(bpy) (O CPh) ](PF ) [4(PF )]. A solution of 958
mg of bpy (6.13 mmol) in 4 mL of toluene was added to a red-orange
solution of 543 mg of Cp Cr (2.98 mmol) in 3 mL of toluene,
4
6
Photolysis experiments were conducted using a standard household
light bulb in a utility light clamp placed 10 cm from the stirring
reaction. Similar results were obtained using either a 100 W
incandescent bulb or a 23 W compact fluorescent bulb.
2
2
2
6
6
2
resulting in a dark-brown solution. A solution of 749 mg of benzoic
acid (6.13 mmol) in 4 mL of toluene was added, resulting in a dark-
blue-black solution. An additional 10 mL of toluene was added, and
the solution was stirred for 20 h. The solvent was removed in vacuo,
and Cr(bpy) (O CPh) was isolated as a crude purple powder of
Spectroscopic and Analytical Methods. UV−visible spectro-
scopic data were collected on a Shimadzu UV 2550 UV−visible
spectrophotometer using sealable UV−visible cells for air-sensitive
1
samples. H NMR spectra were recorded on a Varian Mercury Plus
2
2
2
4
00 spectrometer at ambient temperature. GC/MS analyses of the
sufficient purity for subsequent reactions. A 66 mg sample of
crossover experiment shown in Scheme 4 were conducted on an
Agilent 7890B gas chromatograph with an Agilent 5977 inert chemical
ionization mass detector, utilizing methane as the ionization gas.
Elemental analyses were performed by the Department of Chemistry
Cr(bpy) (O CPh) (0.109 mmol) was dissolved in 3 mL of 2:1
2
2
2
methanol/water, and air was bubbled through the stirring suspension
for 30 min. The reddish-pink precipitate was isolated on a sintered
glass filter and washed with first 2 × 1 mL of MeOH and then 2 × 1
G
DOI: 10.1021/acs.inorgchem.7b03195
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
two crops. Anal. Calcd for C H CrN O : C, 69.33; H, 7.04; N, 3.76.
Table 2. X-ray Crystallographic Data for Complexes 5, 6, and
9
43 53
2
6
Found: C, 69.09; H, 7.14; N, 4.08. UV−visible [Et O; λmax, nm (ε,
2
−1
−1
M
cm )]: 336 (6100), 540 (390). μ(294 K) = 3.78(4) μ .
Synthesis of [Cr( Bu-bpy)(Ph) ][Mg(dpm)(THF) ] (6). To a
solution of 100 mg of Cr( Bu-bpy)(dpm)Cl (0.36 mmol) in 15 mL of
B
t
5
6
9
3
4
t
formula
fw
C H CrN O
C H CrMgN O
C H CrN O
2
2
43
53
2
6
63 90
2
6
35 28
4
THF was added PhMgCl (0.92 mL of a 2.0 M solution in THF, 2.3
mmol). The reaction mixture immediately turned color from green to
dark red. After stirring for 3 days, 200 μL of 1,4-dioxane (2.35 mmol)
was added. After stirring for 1 h, the solvent was removed in vacuo and
745.87
pink, irregular
1047.67
588.61
cryst color,
habit
black, prism
green, irregular
cryst dimens,
mm
0.02 × 0.08 ×
0.18
0.33 × 0.15 × 0.12 0.07 × 0.10 ×
the residue was extracted with 10 mL of Et O and filtered through
0.323
2
Celite. Cooling the filtrate to −30 °C provided 51 mg (13% yield) of
cryst syst
space group
a, Å
monoclinic
monoclinic
triclinic
P1
brown X-ray-quality crystals of 6. Anal. Calcd for C H CrMgN O :
63
90
2
6
P 2 /c
P 2 /c
̅
1
1
C, 72.22; H, 8.66; N, 2.67. Found: C, 59.59; H, 7.69; N, not detected.
13.2567(8)
13.9631(9)
23.1091(15)
90.0
12.0157(12)
19.610(2)
49.596(5)
90.0
10.7073(9)
12.7763(11)
21.3555(14)
87.797(3)
89.996(3)
72.059(4)
2777.1(4)
4
−1
−1
UV−visible [Et O; λmax, nm (ε, M cm )]: 338 (6900), 455 (2700),
2
b, Å
5
41 (2000), 632 (1700). μ(294 K) = 2.95(9) μB.
c, Å
Synthesis of Cr(bpy)(quin)Ph (7). To a suspension of 157 mg
2
α, deg
β, deg
γ, deg
V, Å3
(
0.99 mmol) of CrCl in 10 mL of THF was sequentially added a
3
106.484(1)
90.0
92.328(4)
90.0
solution of 159 mg of bpy (1.02 mmol) in 2 mL of THF, followed by
PhMgCl (1.5 mL of a 2.0 M solution in THF, 3.0 mmol). After stirring
for 20 h, the suspension was filtered through a sintered glass frit. The
red solid was washed with 5 mL of THF and dried under vacuum to
4101.8(4)
4
11677(2)
8
Z
1
1a
Dcalc, g cm−3
give 392 mg of the previously reported Cr(bpy)Ph (THF) as a red
3
1.208
1.192
1.408
powder. To a suspension of this product in 5 mL of THF was added
F000
1588
4528
1224
9
3 mg of quin (0.64 mmol) in 3 mL of THF. After stirring for 23 h,
μ(Mo Kα),
3.26
3.26
4.53
−
1
the brown reaction mixture was filtered through a sintered glass frit.
The brown solid was washed with 3 mL of THF and dried under
vacuum to give 315 mg of 7 (63% yield) as a brown powder. Anal.
Calcd for C H CrN O: C, 73.50; H, 4.78; N, 8.30. Found: C, 68.49;
cm
data images
no.; t, s)
999, 30
3693, 7.5
1383, 60
(
3
1
24
3
2
θmax
52.06°
60.058°
45.1°
−1
−1
H, 5.34; N, 4.99. UV−visible [THF; λmax, nm (ε, M cm )]: 425
reflns measd
32912
209786
45493
(2800).
unique reflns,
Rint
5737, 0.0691
34210, 0.0662
12426, 0.053
Synthesis of Cr(bpy)(quin)Cl . A solution of 159 mg of bpy (1.02
2
mmol) in 2 mL of toluene was added to a suspension of 365 mg of
absorption,
0.900, 0.994
0.7460, 0.6835
24162
0.743, 0.969
9019
CrCl (THF)3 (0.97 mmol) in 10 mL of toluene at ambient
3
Tmin, Tmax
temperature, resulting in a color change from a magenta suspension
to a thick teal-green suspension. An additional 10 mL of toluene was
added, followed by a solution of 151 mg of quin (1.04 mmol) in 2 mL
of toluene, resulting in a green suspension. After stirring for 20 h, the
suspension solvent was filtered through a sintered glass frit. After
drying under vacuum, Cr(bpy)(quin)Cl2 was isolated as a green
powder (317 mg, 77% yield), which was used in subsequent reactions.
obsd data [I > 5737
.00σ(I)]
2
no. of param
469
1599
824
2
R1, wR2 (F , all 0.0777, 0.1185
data)
0.0973, 0.1455
0.114, 0.232
R1, wR2 [F, I > 0.0465, 0.1055
0.0626, 0.1316
0.083, 0.212
2
.00σ(I)]
GOF
max, min peak, 0.690, −0.329
Synthesis of Cr(bpy)(quin)(O CPh) (8). Under nitrogen, 50 mL
1.002
1.069
1.08
2
2
of CH Cl was added to 420 mg of Cr(bpy)(quin)Cl (0.99 mmol)
2
2
2
0.79, −0.495
0.93, −0.43
−
3
and 474 mg of silver benzoate (2.07 mmol). After stirring for 24 h, the
orange suspension was filtered through Celite supported on a sintered
glass frit to remove a white precipitate (presumably AgCl) and give a
brown-yellow filtrate. The solvent was removed in vacuo from the
filtrate, which was dissolved in 5 mL of CH Cl . The addition of 20
e Å
mL of water. After drying under vacuum, 34 mg of [Cr-
bpy) (O CPh) ](PF ) (42% yield) was isolated as a pink powder.
(
2
2
2
6
Anal. Calcd for C H CrF N O P: C, 54.34; H, 3.49; N, 7.46. Found:
C, 54.72; H, 3.99; N, 6.16. UV−visible [NCMe; λmax, nm (ε, M
cm )]: 518 (382).
2
2
34
26
6
4
4
−1
mL of hexanes to the solution induced the precipitation of a gold-
colored product. After the solvent was removed in vacuo, 8 was
isolated as a gold powder (323 mg, 55% yield). Anal. Calcd for
C H CrN O : C, 66.66; H, 4.07; N, 7.07. Found: C, 64.27; H, 4.23;
−1
t
Synthesis of Cr( Bu-bpy)(dpm)Cl . A solution of 253 mg of
2
dipivaloylmethane (dpm or 2,2,6,6-tetramethyl-3,5-heptanedione, 1.38
mmol) in 7 mL of THF was added to a suspension of 502 mg of
CrCl (THF) (1.34 mmol) in 7 mL of THF at ambient temperature.
33 24
3
5
−1
−1
N, 4.56. UV−visible [NCMe; λmax, nm (ε, M cm )]: 419 (1600).
Synthesis of Cr(bpy)(quin) (9). A solution of 49 mg of quin
2
3
3
(
0.34 mmol) in 2 mL of THF was added to 87 mg of
After stirring for 20 h, the solvent was removed in vacuo. The deep-
Cr(bpy)[N(SiMe ) ] (0.16 mmol) in 2 mL of THF. After stirring
green residue was extracted with 20 mL of Et O and filtered through
3 2 2
2
t
for 4 h, 4 mL of hexanes was added to the dark-green suspension, and
the resulting dark-green powder was isolated on a sintered glass filter
and dried under vacuum, providing 72 mg of 9 (0.145 mmol, 88%
yield). X-ray-quality crystals were grown from saturated solutions of
the complex in toluene chilled to −30 °C. Anal. Calcd for
C H CrN O : C, 67.74; H, 4.06; N, 11.28. Found: C, 65.49; H,
Celite on a sintered glass frit. A solution of 361 mg of Bu-bpy (4,4′-di-
tert-butyl-2,2′-dipyridyl, 1.34 mmol) in 5 mL of Et O was added to the
2
filtrate, and the solution was stirred for 20 h. The solvent was removed
t
in vacuo, and Cr( Bu-bpy)(dpm)Cl was isolated as a green powder
2
(
740 mg, 96% yield) of sufficient purity for subsequent reactions. Anal.
Calcd for C H Cl CrN O : C, 60.62; H, 7.54; N, 4.88. Found: C,
5
3
28 20
4
2
2
9
43
2
2
2
−1
−1
8.76; H, 7.26; N, 4.65. UV−visible [THF; λ , nm (ε, M− cm )]:
1
−1
4.59; N, 9.88. UV−visible [toluene; λmax, nm (ε, M cm )]: 335
6300), 356 (5750), 415 (3650), 443 (4100), 624 (1400), 665 (1600).
max
(
47 (5860), 498 (180), 632 (220).
t
Synthesis of Cr( Bu-bpy)(dpm)(O CPh) (5). A suspension of 60
2
2
mg of silver benzoate (0.26 mmol) in 5 mL of THF was added to a
ASSOCIATED CONTENT
■
t
solution of 75 mg of Cr( Bu-bpy)(dpm)Cl (0.13 mmol) in 5 mL of
2
* Supporting Information
S
THF. After stirring for 24 h to produce a pink solution, the solvent was
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03195.
removed in vacuo and the residue was extracted with 6 mL of Et O
and filtered through Celite. Cooling the filtrate to −30 °C provided 21
mg (21% yield) of pink X-ray-quality crystals of 5 over several days in
2
H
DOI: 10.1021/acs.inorgchem.7b03195
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Electrochemistry and spectroelectrochemistry of [Cr-
Forum Article
L.; Brennessel, W. W.; Neidig, M. L. Intermediates and Reactivity in
Iron-Catalyzed Cross-Couplings of Alkynyl Grignards with Alkyl
Halides. J. Am. Chem. Soc. 2017, 139, 6988−7003. (e) Lo, J. C.; Kim,
+
(
bpy) (Ph) ] , photolysis reactions of [Cr(R-bpy)-
2 2
(
C H R) ](X) complexes, bpy bond lengths from X-ray
6
4
2
́
D.; Pan, C.-M.; Edwards, J. T.; Yabe, Y.; Gui, J.; Qin, T.; Gutierrez, S.;
structures of 5, 6, and 9, and UV−visible spectra (PDF)
Giacoboni, J.; Smith, M. W.; Holland, P. L.; Baran, P. S. Fe-Catalyzed
C−C Bond Construction from Olefins via Radicals. J. Am. Chem. Soc.
2017, 139, 2484−2503. (f) Adak, L.; Kawamura, S.; Toma, G.;
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M.; Nakamura, M. Synthesis of Aryl C-Glycosides via Iron-Catalyzed
Cross Coupling of Halosugars: Stereoselective Anomeric Arylation of
Glycosyl Radicals. J. Am. Chem. Soc. 2017, 139, 10693−10701.
Accession Codes
CCDC 1812508−1812510 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(
4) (a) Namba, K.; Wang, J.; Cui, S.; Kishi, Y. Surprisingly Efficient
Catalytic Cr-Mediated Coupling Reactions. Org. Lett. 2005, 7, 5421−
424. (b) Harnying, W.; Kaiser, A.; Klein, A.; Berkessel, A. Cr/Ni-
Catalyzed Vinylation of Aldehydes: A Mechanistic Study on the
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AUTHOR INFORMATION
Corresponding Author
E-mail: kevin.m.smith@ubc.ca.
■
*
4
ORCID
3
Kevin M. Smith: 0000-0003-2802-4939
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Canadian Natural Science and Engineering
Research Council and the Canadian Foundation for Innovation
for supporting this research. K.M.S. and B.E.O. also thank
James Bailey (University of British Columbia, Okanagan) for
help with the electrochemistry and UV−visible spectroelec-
trochemistry and Joseph Clarkson and Laurel Schafer
(
University of British Columbia, Vancouver) for assistance
4
(
4, 6886−6915.
with the GC/MS crossover experiment.
6) (a) Steib, A. K.; Kuzmina, O. M.; Fernandez, S.; Flubacher, D.;
Knochel, P. Efficient Chromium(II)-Catalyzed Cross-Coupling
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