Redox-Switchable Hydroelementation of a Cobalt Complex Supported by a Ferrocene-Based Ligand

Article abstract of DOI:10.1021/acs.organomet.6b00317

The first crystallographically characterized tetrahedral cobalt salen (salen = N,N′-ethylenesalicylimine) complex was synthesized by using a 1,1′-ferrocene derivative, salfen (salfen = 1,1′- di(2,4-di-tert-butyl-6-salicylimine)ferrocene). The complex undergoes two oxidation events at low potentials, which were assigned as ligand centered by comparison to the corresponding zinc complex. The cobalt complex was found to catalyze the hydroalkoxylation of styrenes, similarly to related square planar cobalt salen complexes, likely due to its fluxional behavior in alcoholic solvents. Furthermore, the one-electron-oxidized species was found to be inactive toward hydroalkoxylation. Thus, the hydroalkoxylation reactivity could be turned on/off in situ by redox chemistry.

Full text of DOI:10.1021/acs.organomet.6b00317

Article
pubs.acs.org/Organometallics
Redox-Switchable Hydroelementation of a Cobalt Complex
Supported by a Ferrocene-Based Ligand
Scott M. Shepard and Paula L. Diaconescu*
Department of Chemistry and Biochemistry, University of California, Los Angeles, 607 Charles E Young Drive East, Los Angeles,
California 90095, United States
*
S Supporting Information
ABSTRACT: The first crystallographically characterized tetra-
hedral cobalt salen (salen = N,N′-ethylenesalicylimine) complex
was synthesized by using a 1,1′-ferrocene derivative, salfen
(
salfen = 1,1′- di(2,4-di-tert-butyl-6-salicylimine)ferrocene).
The complex undergoes two oxidation events at low potentials,
which were assigned as ligand centered by comparison to the
corresponding zinc complex. The cobalt complex was found to
catalyze the hydroalkoxylation of styrenes, similarly to related
square planar cobalt salen complexes, likely due to its fluxional
behavior in alcoholic solvents. Furthermore, the one-electron-
oxidized species was found to be inactive toward hydroalkoxylation. Thus, the hydroalkoxylation reactivity could be turned on/
off in situ by redox chemistry.
INTRODUCTION
Despite the prevalence of hydroelementation chemistry, little
if any research has been reported on switchable hydro-
elementation catalysts. Considering that the carbon−carbon
multiple bond is common to all hydroelementation substrates,
reversibly switching the activity of a catalyst toward different
reactions could provide a novel pathway to important
chemicals. Knowles and co-workers recently reported the use
of a photoredox catalyst to promote the intramolecular
■
Hydroelementation, the addition of a hydrogen and a
functional group across an unsaturated bond, is a fundamental
class of organic chemistry reactions with broad applicability for
generating molecular complexity. Additionally, the potential
1
00% atom economy of these reactions makes them attractive1
for both green chemistry and low-cost industrial processes.
Recent advances in the field of transition metal catalysis have
greatly expanded the scope and feasibility of hydroelementation
reactions. Milder reaction conditions, greater regioselectivity,
stereoselectivity, and a wider functional group tolerance than
before are prevalent. This allows the precise formation of
complicated products with vital applications ranging from
28
hydroamination of secondary amines. However, their study
does not contain an investigation of whether the process is
switchable. Another study by Nicewicz and co-workers utilized
29
a photoredox catalyst in the hydrohalogenation of styrenes.
However, this study similarly does not investigate switchability.
Recently, Hiroya and co-workers reported a mild and
functional group tolerant hydrofluorination reaction with a
2
pharmaceuticals to commodity chemical production.
Switchable catalysis is an efficient method that generates
multiple, catalytically active species with different reactivity.
30
3
−25
Co(II) Schiff base complex. This method shows excellent
yields for a large variety of substrates with a relatively
inexpensive catalyst. Furthermore, this versatile system also
leads to hydroalkoxylation in the presence of an alcoholic
Because these species originate from a single precursor and are
readily controlled by external stimuli, rapid production of
molecular complexity can be achieved in one pot. Utilizing
switchable systems for tandem catalysis is advantageous since
problems arising from catalyst compatibility are circumvented.
Switchable catalysis has many parallels in biological systems,
but it has been less studied in the field of chemical synthesis,
largely because of difficulties in regulating multiple catalytically
active species in such a way to generate reactions with high
levels of substrate selectivity. However, recent developments in
31
solvent and intramolecular hydroamination in the presence of
32
a suitable substrate. Dismayed by the lack of switchable
hydroelementation reactions in the literature, we sought to
develop a redox-switchable catalyst for such processes. A
particularly good system to base our work on is the cobalt(II)
salen catalyst employed by Hiroya and co-workers for
hydrofluorination, hydroalkoxylation, and hydroamination.
Not only is this system active toward several different
hydroelementation reactions, but it also utilizes a simple salen
complex. Salfen (salfen = 1,1′- di(2,4-di-tert-butyl-6-
4
8
26
chemical, electrochemical, photochemical, mechanochem-
2
7
7
ical, and allosteric control have led to significant advances
that have made switchable catalysis possible. An important goal
of switchable catalysis is the demonstration of redox switches to
be used for creating structural complexity by combining
Received: April 19, 2016
14,18,24
complementary on/off switches.
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
salicylimine)ferrocene), the Schiff base ferrocene analogue of
solution at ambient temperature produced X-ray quality crystals (0.07
3
3−35
36
1
g, 64.7%). H NMR (CDCl , 500 MHz, 298 K): δ (ppm) −5.77 (s,
salen,
was initially synthesized by Arnold and co-workers
3
37
38
18H, C(CH ) ), 1.25 (s, 4H, Cp-H), 7.59 (s, 12H, B(C H (CF ) ) ),
and has been used in complexes of cerium and indium for
the catalytic ring-opening polymerization of cyclic esters. In this
work, we report the synthesis and characterization of a
cobalt(II) salfen complex, Co(salfen), and an investigation of
its activity toward redox-switchable hydroelementation reac-
tions. The system was found to be active toward the
hydroalkoxylation of olefins in its reduced state and inactive
in its oxidized state, producing a redox-reversible on/off switch.
The complex was not found to be active toward hydro-
amination of olefins or reactions of alkynes. During this
investigation, it was necessary to synthesize and characterize the
corresponding zinc(II) complex, Zn(salfen), in order to
understand the redox behavior of Co(salfen).
3
3
6
3
3 2 4
9
.30 (s, 18H, C(CH ) ), 55.60 (s, 2H, aromatic), 61.43 (s, 2H,
3 3
9
aromatic). F NMR (CDCl , 400 MHz, 298 K): δ (ppm) −62.59
(
CoO N BF C H ): C, 57.09; H, 3.81; N, 1.71. Found: C, 57.22;
H, 3.66; N, 1.54. μeff = 4.80 μB.
Co(salfen)]4 (4). Compound 2 (0.1012 g, 0.14 mmol) was
dissolved in 5 mL of THF. To this solution was added 1 mL of
MeOH, and the solution was stirred for 4 h. All volatiles were removed
under reduced pressure to yield a red solid. The product was purified
by layering a concentrated toluene solution of the compound with
MeOH. X-ray quality crystals were obtained from prolonged standing
3
BArF ). Anal. Calcd for [Co(salfen)][BAr
F
]
(Fe-
4
4
2
2
24 78 62
[
1
of this toluene/MeOH solution at ambient temperature. H NMR
(
(
CDCl , 500 MHz, 298 K): δ (ppm) −8.24 (s, 4H, NCH), −5.11
3
s, 36H, C(CH ) ), −3.83 (s, 36H, C(CH ) ), −2.89 (s, 4H, N
3
3
3 3
CH), 0.31 (s, 72H, C(CH ) ), 8.94 (s, 8H, Cp-H), 12.86 (s, 8H, Cp-
3
3
EXPERIMENTAL SECTION
H), 49.55 (s, 4H, aromatic), 54.94 (s, 4H, aromatic), 61.43 (s, 4H,
aromatic), 64.26 (s, 4H, aromatic). Anal. Calcd for 4
■
General Considerations. All reactions were performed using
standard Schlenk techniques or in an MBraun drybox under a nitrogen
atmosphere, unless otherwise noted. All glassware and Celite were
stored in an oven at >425 K overnight before being brought into the
drybox. Solvents, except for methanol, were purified by the method of
(Fe Co O N C160H200): C, 68.09; H, 7.14; N, 3.43. Found: C,
4 4 8 8
8.96; H, 6.49; N, 3.43.
Zn(salfen) (5). Compound 1 (0.09 g, 0.14 mmol) was dissolved in
mL of THF and added dropwise to a stirring slurry of potassium
6
5
39
hydride (0.01 g, 0.30 mmol) in THF. The dark red solution was stirred
for 2 h before filtering it through Celite. Then, all volatiles were
removed under reduced pressure. The resulting red solid was washed
with hexanes to remove any starting material. The solid was then
Grubbs and transferred to the glovebox without exposure to air.
Methanol was purified by stirring over calcium hydride overnight
followed by distillation under a nitrogen atmosphere and transferred
into the drybox without exposure to air. NMR solvents were obtained
from Cambridge Isotope Laboratories, degassed, and stored over
activated molecular sieves prior to use. NMR spectra were recorded at
ambient temperature on Bruker AV-300, AV-400, AV-500, and DRX-
dissolved in THF, and ZnCl (0.02 g, 0.14 mmol) was added to the
2
solution. The red solution was stirred for 1 h before the solvent was
removed under reduced pressure. The red solid was dissolved in
toluene and filtered through Celite to give a red solution. Removal of
5
00 spectrometers. Proton chemical shifts are given relative to residual
all volatiles under reduced pressure afforded the pure product as a dark
solvent peaks. Fluorine chemical shifts are given relative to freon.
1
Magnetic susceptibility measurements were performed in either
red solid (0.09 g, 95.9%). H NMR (CDCl
3
, 500 MHz, 298 K): δ
), 1.41 (s, 18H, C(CH ), 3.96−4.14
(d, 2H, Cp-H), 4.39−4.64 (d, 2H, Cp-H), 7.02−7.03 (d, 2H,
4
0,41
CD Cl2 or CDCl3 using the Evans method.
UV−vis spectra
(ppm) 1.34 (s, 18H, C(CH
3
)
3
)
3 3
2
were obtained on an Agilent Cary 5000 UV−vis−NIR spectrometer.
Samples were prepared with dry degassed THF under an inert
atmosphere and loaded into a quartz cuvette sealed with a Teflon
1
3
aromatic), 7.48−7.49 (d, 2H, aromatic), 8.49 (s, 2H, NCH).
NMR (CDCl , 500 MHz, 298 K): δ (ppm)) 29.5 (CH ), 31.4 (CH
34.1 (CCH ), 35.8 (CCH
C
),
3
3
3
3
6
valve. H (salfen), 1, was prepared according to literature procedures.
3
3
), 60.7 (Cp), 71.7 (Cp), 72.8 (Cp CN),
2
All other reagents were acquired from commercial sources in the
highest available purity and, unless otherwise stated, used as received.
Elemental analyses were performed on an Exeter Analytical Inc. CE-
106.3 (aromatic), 117.9 (aromatic), 128.9 (aromatic), 130.3
(aromatic), 142.4 (aromatic), 166.3 (NCH), 170.1 (CO). Anal.
Calcd for Zn(salfen) (FeZnO N C H50): C, 67.47; H, 7.08; N, 3.93.
2 2 40
4
40 elemental analyzer.
Co(salfen) (2). Compound 1 (0.29 g, 0.45 mmol) was dissolved in
Found: C, 66.88; H, 6.98; N, 3.90.
F
[Zn(salfen)][BAr
dissolved in 3 mL of dichloromethane. To this red solution was added
a blue solution of acetylferrocenium [BArF
] (0.007 g, 0.006 mmol) in
dichloromethane. The resulting black solution was stirred for 30 min
before the solvent was removed under reduced pressure. The black
] (6). Compound 2 (0.004 g, 0.06 mmol) was
4
5
mL of THF and added dropwise to a stirring slurry of potassium
hydride (0.04 g, 0.98 mmol) in THF. The dark red solution was stirred
for 2 h before filtering through Celite. Then, all volatile substances
were removed under reduced pressure. The resulting red solid was
washed with hexanes to remove any starting material. The solid was
4
solids were then rinsed with hexanes to afford the product as a black
solid (0.008 g, 90.4%). H NMR (CDCl
1
then dissolved in THF, and CoCl (0.06 g, 0.45 mmol) was added to
3
, 500 MHz, 298 K): δ (ppm)
2
the solution. The red solution was stirred for 1 h before the volatiles
were removed under reduced pressure. The red solid was dissolved in
toluene and filtered through Celite to give a red solution. Evaporation
of the toluene under reduced pressure afforded the pure product as a
dark red solid (0.28 g, 87.7%). X-ray quality crystals were obtained
−2.70, −2.36, 0.06, 1.21, 2.36, 2.56, 3.47, 6.89, 7.72, 13.90, 15.52, 25.0.
F
4
Anal. Calcd for [Zn(salfen)][BAr
] cocrystallized with one molecule
74BF24): C, 54.90; H, 3.97; N, 1.78. Found:
C, 54.26; H, 3.60; N, 1.17.
[Zn(salfen)] (7). Compound 5 (0.05 g, 0.07 mmol) was dissolved
Ac
of Fc (Fe ZnO N C H
2 3 2 84
4
from the slow evaporation of a benzene solution at ambient
in toluene, and the solution concentrated. This was layered with
methanol, and the product formed red crystals upon standing.
1
temperature. H NMR (CDCl , 500 MHz, 298 K): δ (ppm) −8.94
3
(
9
s, 1H, NCH), −3.62 (s, 18H, C(CH ) ), −2.90 (s, 1H, NCH),
Recrystallizing in this manner once more afforded a pure product and
3
3
1
.90 (s, 18H, C(CH ) ), 10.86 (s, 4H, Cp-H), 17.65 (s, 4H, Cp-H),
X-ray quality crystals (0.04 g, 82.1%). H NMR (CDCl
3
, 500 MHz,
), 1.28 (s, 36H, C(CH ),
), 1.57 (s, 36H, C(CH ), 3.67 (s, 8H, Cp-H),
3
3
5
3.70 (s, 2H, aromatic), 60.97 (s, 2H, aromatic). Anal. Calcd for
298 K): δ (ppm) 1.13 (s, 36H, C(CH
1.37 (s, 36H, C(CH
3
)
3
)
3 3
Co(salfen) (FeCoO N C H ): C, 68.09; H, 7.14; N, 3.97. Found: C,
3
)
3
)
3 3
2
2 40 50
6
8.77; H, 6.61; N, 3.72. μ = 4.34 μB.
3.93 (s, 8H, Cp-H), 4.36 (s, 8H, Cp-H), 4.50 (s, 8H, Cp-H), 4.74 (s,
8H, Cp-H), 6.67 (s, 4H, aromatic), 6.86 (s, 4H, aromatic), 7.50 (s, 8H,
aromatic), 8.37 (s, 8H, NCH). Anal. Calcd for [Zn-
(salfen)] (toluene) (Fe Zn O N C H ): C, 69.57; H, 7.22; N,
4 3 4 4 8 8 181 224
eff
F
[
Co(salfen)][BAr ] (3). Compound 2 (0.04 g, 0.6 mmol) was
4
dissolved in 3 mL of dichloromethane. To this red solution was added
a blue solution of acetylferrocenium [BAr ] (Ar = 3,5-(CF ) -C H ,
F
F
4
3
2
6
3
0.07 g, 0.06 mmol) in dichloromethane. The resulting black solution
3.59. Found: C, 70.22; H, 7.21; N, 3.18.
was stirred for 30 min before all volatiles were removed under reduced
pressure. The black solid was then washed with hexanes until the
solution appeared colorless. The product was crystallized by layering a
concentrated toluene solution with hexanes. Prolonged standing of this
General Procedure for Hydroalkoxylation Reactions. Com-
pound 2 (0.01 mmol) was dissolved in 1 mL of dichloromethane. To
this solution was added trimethoxybenzene (0.15 mmol) as an internal
standard as well as N-fluoro-2,4,6-trimethylpyridine tetrafluoroborate
B
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1. Synthesis of Compounds 2−7
(
0.22 mmol), 1,1,3,3-tetramethyldisiloxane (0.22 mmol), the unsatu-
potentiostat and recorded with CH Instruments software (version
13.04) with data processing on Origin 9.2. All potentials are given with
respect to the ferrocene/ferrocenium couple.
rated substrate (0.20 mmol), and the appropriate alcohol (0.22 mmol).
The reaction was allowed to stir until completion and monitored by
1
H NMR spectroscopy, by collecting aliquots of the reaction mixture.
X-ray Crystallography. X-ray quality crystals were obtained from
various concentrated solutions placed in a −40 °C freezer in the
glovebox unless otherwise specified. Inside the glovebox, the crystals
were coated with oil (STP oil treatment) on a microscope slide, which
was brought outside the glovebox. The X-ray data collections were
carried out on a Bruker SMART 1000 single-crystal X-ray
diffractometer using Mo Kα radiation and a SMART APEX CCD
detector. The data were reduced by SAINTPLUS, and an empirical
absorption correction was applied using the package SADABS. The
structure was solved and refined using SHELXTL (Bruker 1998,
SMART, SAINT, XPREP, AND SHELXTL, Bruker AXS Inc.,
Madison, WI, USA). Tables with atomic coordinates and equivalent
isotropic displacement parameters, with all the distances and angles
and with anisotropic displacement parameters, are listed in the
Supporting Information.
The volatile substances were removed from the aliquots under reduced
pressure, and the solids were dissolved in CDCl . Product formation
and yield were determined by H NMR relative integration to
trimethoxybenzene.
General Procedure for Redox-Switchable Hydroalkoxyla-
tion. Compound 2 (0.01 mmol) was dissolved in 1 mL of
dichloromethane. To this solution was added trimethoxybenzene
3
1
(
0.15 mmol) as an internal standard as well as N-fluoro-2,4,6-
trimethylpyridine tetrafluoroborate (0.22 mmol), 1,1,3,3-tetramethyl-
disiloxane (0.22 mmol), the unsaturated substrate (0.20 mmol), and
the appropriate alcohol (0.22 mmol). The reaction was monitored by
1
H NMR spectroscopy, by collecting aliquots of the reaction mixture
every hour. The volatile substances were removed from the aliquots
under reduced pressure, and the solids were dissolved in CDCl . After
2
3
h, acetylferrocenium BArF (0.01 mmol) was added to the reaction
4
mixture. After 4 h, cobaltocene (0.011 mmol), N-fluoro-2,4,6-
trimethylpyridine tetrafluoroborate (0.22 mmol), and 1,1,3,3-tetrame-
thyldisiloxane (0.22 mmol) were added to the reaction mixture.
Electrochemical Studies. Cyclic voltammetry studies were
carried out in a 20 mL scintillation vial with electrodes fixed in
position by a rubber stopper. The electrolyte used was 0.10 M
tetrabutylammonium hexafluorophosphate, which was recrystallized
twice from ethanol before use. The solvent used was dichloromethane.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Cobalt and Zinc
Complexes. Compound Co(salfen) (2) was prepared by salt
metathesis between the potassium salt of H (salfen) (1) and
2
CoCl (Scheme 1). Compound 2 is paramagnetic, as expected
2
for a cobalt(II) complex. Its solid-state molecular structure was
determined by single-crystal X-ray diffraction (Figure 1), which
indicates that the complex adopts a pseudotetrahedral geometry
about the cobalt center. The N(1)−Co(2)−O(1) angle of
2
A glassy carbon working electrode (planar circular area = 0.071 cm ), a
2
platinum reference electrode (planar circular area = 0.031 cm ), and a
silver-wire pseudoreference electrode were used and purchased from
CH Instruments. Before each cyclic voltammetry experiment, the
working and auxiliary electrodes were polished with an aqueous
suspension of 0.05 μm alumina on a Microcloth polishing pad. Cyclic
voltammograms were acquired with a CH Instruments CHI630D
9
2.81(9)° and N(2)−Co(2)−O(2) angle of 93.53(9)° are
nearly 90°, as expected for a square planar complex, and agree
closely with previously reported square planar cobalt salen
42
complexes. However, these two planes are offset by an angle
C
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
F
Figure 2. Solid-state crystal structure of [Co(salfen)][BAr ] (3) with
4
Figure 1. Solid-state molecular structure of Co(salfen) (2) with
thermal ellipsoids set at 50% probability. Hydrogen atoms and solvent
molecules were omitted for clarity. Selected distances (Å) and angles
thermal ellipsoids set at 50% probability. Hydrogen atoms, solvent
F
4
molecules, and the [BAr ] ion were omitted for clarity. Selected
distances (Å) and angles (deg): O(1)−Co(1), 1.8845(13); O(2)−
Co(1), 1.8940(13); N(1)−Co(1), 1.9984(15); N(2)−Co(2),
(
1
deg): N(1)−Co(2), 2.006(2); N(2)−Co(2), 2.009(2); O(1)−Co(2),
.910(2); O(2)−Co(2), 1.912(2); N(1)−Co(2)−O(1), 92.81(9);
2.0028(15); O(1)−Co(1)−O(2), 112.27(6); O(1)−Co(1)−N(1),
94.09(6); O(1)−Co(1)−N(2), 129.37(6); O(2)−Co(1)−N(2),
92.76(6); O(2)−Co(1)−N(1), 122.74(6); N(1)−Co(1)−N(2),
108.63(6).
O(1)−Co(2)−O(2), 112.37(9); O(2)−Co(2)−N(2), 93.53(9);
N(2)−Co(2)−N(1), 107.16(9); O(1)−Co(2)−N(2), 125.50(9);
N(1)−Co(2)−N(2), 128.96(9).
of 75.31°. The angle between these two planes in analogous
for the oxidized complex is 0.76, indicating a similar distortion
from an ideal tetrahedron as for 2. The ferrocene moiety in 3 is
more distorted than in 2, with the distance from the Cp
centroids to iron of 1.70 Å and an angle between them of
173.10° compared to 1.64 Å and 175.42° in 2.
43
complexes is around 15°. Calculating the τ parameter for the
4
complex yields a value of 0.75, indicating a fairly distorted
4
4
tetrahedron. The two nitrogen cobalt distances are nearly
identical, with a N(1)−Co(2) distance of 2.006(2) Å and a
N(2)−Co(2) distance of 2.009(2) Å. Similarly, the oxygen
cobalt distances are nearly identical, with an O(1)−Co(2)
distance of 1.910(2) Å and an O(2)−Co(2) distance of
Compound 3 is also paramagnetic. The nature of this
oxidation was probed by determining the complex’s magnetic
susceptibility, 4.80 μ , using the Evans method. If the oxidation
B
1
.912(2) Å. All four distances are slightly longer than previously
to 3 resulted in a one-electron oxidation of the iron center, the
complex would exhibit a theoretical spin-only magnetic
42
reported for analogous complexes. Four-coordinate cobalt(II)
salen complexes typically exhibit a square planar geometry, and
a search of the CCSD did not result in any examples of
tetrahedral cobalt salen complexes. However, the literature
contains one report of tetrahedral coordination in cobalt salen
nanoparticles on the basis of EXAFS. This unusual geometry
suggests a unique steric and electronic environment. The
electronic structure of the compound was investigated by the
susceptibility of 4.24 μ for the noninteracting S = 3/2 (cobalt)
B
and S = 1/2 (iron) spins. For comparison, the theoretical spin-
only magnetic susceptibility for an S = 2 system, such as high-
spin cobalt(III), would be 4.90 μ . This suggests that 3 likely
B
45
contains one more unpaired electron than 2 but does not allow
us to assign the first oxidation event unambiguously.
Despite magnetic data showing an additional unpaired
electron in compound 3 compared with 2, we could not
assign, on this basis alone, the oxidation states of iron and
cobalt in 3. Thus, we synthesized the zinc complex
corresponding to 2, Zn(salfen) (5), by an analogous route
Evans method, which yielded a magnetic susceptibility of 4.34
7
μ , suggesting an S = 3/2 d cobalt, as expected for a tetrahedral
B
geometry. Previously reported square planar cobalt(II) salen
complexes exhibit magnetic susceptibilities near 1.7 μ ,
B
7
42,46−48
indicative of low-spin, S = 1/2 d cobalt.
This unusual
(Scheme 1). Compound 5 was also found to undergo a one-
F
geometry is likely due to ferrocene’s torsional flexibility relative
electron oxidation in the presence of acetylferrocenium [BAr ]
4
49
to common salen ligand backbones.
Compound 2 was found to undergo a one-electron oxidation
to produce a stable black compound, 6. Unfortunately, 5 and 6
eluded X-ray crystallographic characterization. However, the
zinc complex was found to crystallize in the presence of
methanol to form compound 7. X-ray-quality crystals were
obtained for 7 revealing a tetrameric structure analogous to 4.
This suggests that the other zinc complexes likely exhibit
geometries comparable to their cobalt analogues.
F
F
in the presence of acetylferrocenium [BAr ] (Ar = 3,5-
4
(
CF ) -C H ) to produce the stable and isolable species
3 2 6 3
F
[
Co(salfen)][BAr ] (3, Scheme 1). Complex 3 could be
4
reduced back to 2 by the addition of cobaltocene. The solid-
state molecular structure of this black oxidized species was
determined by single-crystal X-ray diffraction (Figure 2).
Similar to the reduced compound 2, a pseudotetrahedral
coordination environment is observed around the cobalt center
While studying the reactivity of 2, we noticed that the
1
corresponding H NMR spectra began to show decomposition.
The nature of this decomposition was investigated and found to
be the result of methanol present in the drybox atmosphere, a
reagent used for hydroalkoxylation reactions (see below).
Furthermore, 2 was found to convert cleanly to a new product,
4, when exposed to methanol and crystallized in its presence.
Single-crystal X-ray diffraction studies revealed the solid-state
molecular structure of this species to be a tetrameric isomer in
with the primary structural difference being the presence of the
F
[
BAr ] counterion. The nitrogen−cobalt and oxygen−cobalt
4
distances are slightly shorter in 3 than in 2. The angles around
the cobalt center remain relatively unchanged from the reduced
species, but the angle between the two nitrogen−cobalt−
oxygen planes is slightly larger in 3, at 77.18°. The τ parameter
4
D
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
which the two phenolate arms of each ferrocene are
coordinated to two different cobalt centers (Figure 3).
these two planes is somewhat larger, 81.70°, in the tetrameric
structure. As a result, the τ parameter for the tetrameric
4
structure is 0.82, indicating a geometry significantly closer to a
tetrahedron than in the monomer. The nitrogen−cobalt and
oxygen−cobalt distances remain almost unchanged from the
monomeric structure. Variable-temperature NMR spectroscopy
was used to probe whether this tetramer can convert to the
monomeric compound with heating. Several peaks in the
spectrum coalesce at 80 °C; however, when the sample was
returned to ambient temperature, the unchanged tetramer
spectrum was observed (Figure S3). This result suggests that
the tetrameric structure is more thermodynamically stable. This
greater stability presumably arises from less steric congestion,
which also allows for a geometry closer to a tetrahedron.
Redox Behavior of Cobalt and Zinc Complexes. Cyclic
voltammetry (CV) was employed to investigate the redox
behavior of the synthesized compounds. The CV for 2 showed
a reversible oxidation event at 0.22 V and a second quasi-
+
reversible oxidation at 0.93 V vs Fc/Fc . Compound 5 displays
a reversible oxidation at 0.19 V and a second quasi-reversible
+
oxidation at 0.89 V vs Fc/Fc . As zinc does not have any easily
accessible redox chemistry, we can assign both of these
oxidation events as being ligand based. Astoundingly, the cyclic
voltammograms for the cobalt and zinc complexes are nearly
identical, suggesting that the cobalt(II)/cobalt(III) couple is
not observed at these low potentials. Instead these oxidation
events must correspond to the iron(II)/iron(III) couple or the
formation of organic ligand based radicals. However, the
specific assignment of these oxidation processes is ambiguous.
The one-electron-oxidized species 3 and 6 are notably EPR
silent at room temperature. The fact that ferrocenium EPR
signals are usually observable only at liquid helium temper-
Figure 3. Solid-state crystal structure of [Co(salfen)]4 (4) with
thermal ellipsoids set at 50% probability. Hydrogen atoms and solvent
molecules were omitted for clarity. Selected distances (Å) and angles
(
deg): O(1)−Co(1), 1.9005(15); O(2)−Co(1), 1.9102(16); N(1)−
Co(1), 1.9866(18); N(2)−Co(1), 1.9967(18); O(1)−Co(1)−O(2),
121.51(7); O(1)−Co(1)−N(1), 95.57(7); O(1)−Co(1)−N(2),
111.76(7); N(1)−Co(1)−N(2), 122.44(7); O(2)−Co(1)−N(2),
95.53(7).
50
atures coupled with the reversibility of the oxidations suggests
that the first oxidation corresponds to the oxidation of the
ferrocene moiety. However, UV−vis spectroscopy was used to
investigate 2, 3, 5, and 6, and it indicates that all the
compounds have similar absorption spectra (Figures S30−S32).
If the first oxidation event corresponded to iron oxidation, the
characteristic blue color of ferrocenium would be expected,
51
manifested as an absorption near 600 nm. The lack of this
spectral feature for 3 and 6 instead suggests the formation of an
organic radical. The formation of phenoxyl radicals upon
oxidation of analogous cobalt salen complexes is well
46,52
known.
However, no activity was observed between 3
and the common radical traps TEMPO, diphenyl disulfide, and
tributyltin hydride (Figures S11−S13).
The redox behavior of 1 was also investigated to ensure that
these two remarkably similar cyclic voltammetry profiles are not
the result of decomposition, although in neither case was the
electrolyte found to decompose the complexes into the free
ligand. Furthermore, the CV of the free ligand shows two
similar oxidation events; however the first event appears almost
Figure 4. Solid-state crystal structure of [Zn(salfen)]4 (7) with
thermal ellipsoids set at 50% probability. Hydrogen atoms and solvent
molecules were omitted for clarity. Selected distances (Å) and angles
2
00 mV lower than for the cobalt or zinc complexes (Figure
S29). This supports the idea that both of these oxidations are
related to the supporting ligand and that these cyclic
voltammograms are not the result of decomposition.
(
1
deg): Zn(1)−N(1), 2.009(2); Zn(1)−N(2), 2.009(2); Zn(1)−O(1),
.906(2); Zn(1)−O(2), 1.924(2); N(1)−Zn(1)−O(1), 96.27(7);
O(1)−Zn(1)−O(2), 112.201(7); N(2)−Zn(1)−N(1), 122.31(8);
N(1)−Zn(1)−O(2), 111.78(7); O(1)−Zn(1)−O(2), 120.18(7);
N(2)−Zn(1)−O(2), 95.87(7).
Furthermore, 1 was investigated by UV−vis spectroscopy
F
4
before and after an in situ oxidation with [AcFc][BAr ],
resulting in similar spectra to those of 2, 3, 5, and 6 (Figure
S32).
Hydroalkoxylation of Olefins. Inspired by the work of
Hiroya and co-workers, compound 2 was investigated for
catalytic activity toward the hydroalkoxylation of olefins in the
The complex retains a pseudotetrahedral coordination
environment at each cobalt center. Like the monomeric
structure, the angle between the nitrogen and oxygen on the
same phenolate arm is nearly 90°, although the angle between
E
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Chart 1. Products of the Hydroalkoxylation Reactions
1
, entries 6−9) were performed in order to confirm that all
reagents were necessary for the reaction to proceed. In addition
to the standard control experiments (Table 1, entries 6−8), 2
was replaced with an organic base, triethylamine (Table 1, entry
9), as the combination of a silane and halogenation reagent was
shown to catalyze intramolecular hydroalkoxylation. Interest-
ingly, the tetrameric cobalt complex, 4, also showed activity
toward hydroalkoxylation, although at a reduced rate compared
to the monomeric compound (Table 1, entry 10). When using
the zinc complex 5 in place of 2, no catalytic activity was
observed, demonstrating that cobalt is necessary for catalysis
rather than just the ferrocene-based ligand or a generic Lewis
acid.
Figure 5. Cyclic voltammograms of Co(salfen), 2, and Zn(salfen), 5,
n
recorded in dichloromethane with 0.1 M [ Bu N][PF ] as the
4
6
+
electrolyte. Both CVs are referenced to the Fc/Fc couple. The y axes
31
of the graphs were normalized to similar dimensions.
presence of a silane and an electrophilic fluorinating agent in
trifluorotoluene. With comparable conditions to previous work,
the system was shown to be active toward the addition of
methanol to 4-methoxystyrene as a model substrate (eq 1),
reaching completion in 24 h.
A mechanism for the cobalt salen catalyzed hydroalkox-
ylation was proposed by Hiroya on the basis of deuterium
31
labeling and radical clock experiments (Scheme S1). The
proposed mechanism is consistent with earlier work on cobalt
salen catalyzed hydroalkoxylation done by Carreira, in
suggesting the formation of a cobalt hydride species capable
53
of undergoing migratory insertion with an olefin substrate.
The proposed mechanism notably occurs through a cobalt(III)
hydride that would necessitate a five-coordinate metal center.
Therefore, we propose that 2 is fluxional in the presence of
alcohols, allowing it to adopt a square pyramidal geometry and
access the cobalt(III) oxidation state. The formation of the
tetrameric species [Co(salfen)]4 (4) in the presence of
methanol supports this proposal. This rearrangement neces-
sitates a fluxional ligand behavior and is only observed in the
presence of methanol; this fact may also explain the complex’s
activity toward hydroalkoxylation but not toward other types of
hydroelementation reactions.
Optimization of reaction conditions (Table 1) led to the
selection of dichloromethane as the optimal solvent, reducing
Table 1. Optimization and Control Reactions for the
a
Hydroalkoxylation of 4-Methoxystyrene
entry
silane
fluorinating agent catalyst
solvent
conv
1
2
3
4
5
6
7
8
9
TMDSO
TMDSO
TMDSO
TMDSO
TMDSO
NFPBF4
NFPBF4
NFPBF4
NFPBF4
NFPBF4
NFPBF4
NFSI
2
2
2
2
2
2
2
C H CF
3
28%
14%
27%
56%
99%
0%
6
5
C H CH
3
6
5
(CH ) O
2
4
CH OH
3
The proposed mechanism notably invokes cobalt(III)
intermediates; yet this oxidation state does not appear
accessible under cylic voltammetry conditions. Several attempts
to generate cobalt(III) complexes were undertaken in order to
test this mechanistic proposal. Cobalt(III) salen complexes are
typically generated by the aerobic oxidation of the correspond-
CH Cl2
2
CH Cl2
2
TMDSO
TMDSO
TMDSO
TMDSO
TMDSO
CH Cl2
0%
2
NFPBF4
NFPBF4
CH Cl2
0%
2
Et N
CH Cl2
0%
3
2
10
1
NFPBF4
4
5
CH Cl2
22%
0%
2
54
ing cobalt(II) complex in the presence of a nucleophile. Such
1
NFPBF4
CH Cl2
2
attempts with 2 were unsuccessful, resulting in free ligand due
to hydrolysis. Chemical oxidation of 2 in the presence of alkyl
phosphines, imidazole, or acetic acid resulted in either a
complex mixture of products or free ligand (Figures S14−S16).
In contrast to direct oxidation, attempts were also made to
generate a cobalt(III) complex by first reducing 2 to cobalt(I)
since it is known that cobalt(I) compounds undergo oxidative
addition of alkyl halides to generate cobalt(III) alkyl
complexes. Reduction with either sodium metal or KC8
afforded a black compound presumed to be a cobalt(I)
complex (Figure S17). However, this compound did not
undergo any reaction in the presence of alkyl or aryl halides
(Figures S18 and S19). Furthermore, a reaction using a
a
1
Reaction time of 6 h; conversion determined by H NMR integration
versus trimethoxybenzene as an internal standard. TMDSO = 1,1,3,3-
tetramethyldisiloxane, NFPBF = N-fluoro-2,4,6-trimethylpyridinium
tetrafluoroborate, NFSI = N-fluorobenzenedisulfonimide.
4
reaction times to approximately 6 h. The system demonstrated
similar activity toward a variety of styrene derivatives (Chart 1),
reaching near-quantitative yields as determined by NMR
spectrocopy in a matter of hours. However, little to no activity
was observed for a variety of other olefin substrates including
alkyl olefins and norbornene derivatives, indicating a need for
activated styrene monomers. Various control reactions (Table
55
F
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
cobalt(III) salt, Na [Co(CO ) ], as a starting material was
metal and the first crystallographically characterized cobalt
example. Magnetic data confirm the presence of three unpaired
electrons consistent with a tetrahedral d cobalt(II) species.
3
3 3
unsuccessful (Figure S20). These results are expected since
56
7
cobalt(III) is substitutionally inert and, therefore, rarely used
as a starting material. These efforts were further hindered by
the insolubility of most simple cobalt(III) salts in common
organic solvents.
Furthermore, the compound has two oxidations at potentials
low enough to be observed by cyclic voltammetry. Surprisingly,
the corresponding zinc complex, which also exhibits a
pseudotetrahedral geometry, displays a nearly identical cyclic
voltammogram to that of Co(salfen). Accordingly, both of the
observed oxidations can be assigned to the supporting ligand.
Although the oxidation from cobalt(II) to cobalt(III) is
typically easily accessed in square planar cobalt salen
complexes, our studies indicate that this process is not facile
in the present case. The unique torsional flexibility and redox
activity of the ferrocene backbone result in a compound vastly
different from previously reported cobalt salen complexes.
Surprisingly, this cobalt complex is able to catalyze the
hydroalkoxylation of olefins similar to other cobalt salen
complexes, albeit with a much smaller substrate scope. As this
reaction was proposed to involve the formation of a cobalt
hydride, the complex must exhibit some fluxional behavior in
solution to open up a coordination site necessary for the
hydride ligand. The possibility of this fluxional behavior is
evidenced by the formation of a tetrameric structure in the
presence of methanol. Furthermore, oxidation and reduction of
the supporting ligand modulates reactivity toward hydro-
alkoxylation in an on/off manner, where the oxidized
compound is inactive.
In addition, cobalt(II) complexes are known to begin cobalt-
mediated radical polymerization by generating a cobalt(III)
57
hydride, which can be trapped by olefins. However, heating 2
in the presence of azo-bis-isobutyronitrile and a styrene
afforded no reaction (Figure S21), producing neither a
cobalt(III) alkyl complex nor a polymeric material. The lack
of success in generating a cobalt(III) complex further supports
the assignment of the oxidation events described previously as
ligand based.
In contrast to 2, the oxidized compound, 3, was found to be
inactive toward hydroalkoxylation even for styrene substrates.
This promising finding coupled with the chemical reversibility
of the compound’s oxidation and reduction suggested that the
cobalt complex could be used as an in situ on/off switch. This
switch was successfully demonstrated with the hydroalkox-
ylation of 4-methoxystyrene, beginning with compound 2
(Figure 6). The activity was completely halted upon oxidation
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00317.
NMR spectra and cyclic voltammetry data (PDF)
Crystallographical data (CIF)
1
Figure 6. Plot of conversion (as determined by H NMR integration
versus trimethoxybenzene as an internal standard) versus time of the
AUTHOR INFORMATION
Corresponding Author
*E-mail (P. L. Diaconescu): pld@chem.ucla.edu.
■
redox switch for the hydroalkoxylation of 4-methoxystyrene. [AcFc]-
F
[
BAr ] was added after 2 h, and cobaltocene was added after 4 h.
4
Notes
F
4
The authors declare no competing financial interest.
with acetylferrocenium [BAr ]. Subsequent reduction with
CoCp and the addition of another equivalent of the silane and
2
ACKNOWLEDGMENTS
fluorinating agents nearly restored full activity. If additional
silane and fluorinating agent were not added, however, the
reaction stalled out sooner. If the two reagents were not added
immediately after reduction, they could be added several hours
later, when the reaction had stalled, to increase conversion to
some degree. Notably, in neither case did the reaction reach the
near-quantitative yields observed in the absence of the redox
switch. This observation can be attributed to some decom-
position of the catalyst or side reactions caused by the in situ
oxidation and reduction evidenced by the appearance of some
■
We thank Mr. Kevin Miller for obtaining single crystals of
Co(salfen). This work was supported by NSF, Grant 1362999
to P.L.D. and CHE-1048804 for NMR spectroscopy, and the
John Simon Guggenheim Memorial Foundation.
REFERENCES
■
(
1) Beller, M.; Seayad, J.; Tillack, A.; Jiao, H. Angew. Chem., Int. Ed.
2004, 43, 3368.
(2) Muller, T. E.; Hultzsch, K. C.; Yus, M.; Foubelo, F.; Tada, M.
̈
Chem. Rev. 2008, 108, 3795.
1
unknown H NMR signals after the reduction took place
(Figure S10).
(3) Blanco, V.; Leigh, D. A.; Marcos, V. Chem. Soc. Rev. 2015, 44,
5
(
341.
4) Gregson, C. K. A.; Gibson, V. C.; Long, N. J.; Marshall, E. L.;
Oxford, P. J.; White, A. J. P. J. Am. Chem. Soc. 2006, 128, 7410.
5) Tennyson, A. G.; Lynch, V. M.; Bielawski, C. W. J. Am. Chem. Soc.
010, 132, 9420.
6) Sußner, M.; Plenio, H. Angew. Chem., Int. Ed. 2005, 44, 6885.
CONCLUSIONS
■
Incorporating a ferrocene moiety into the backbone of a cobalt
salen, a well-studied class of catalysts, produced a complex with
unusual steric, electronic, and electrochemical properties.
Co(salfen) exhibits a pseudotetrahedral geometry at the cobalt
center, an unusual geometry among salen complexes of any
(
2
(
̈
(7) Yoon, H. J.; Kuwabara, J.; Kim, J.-H.; Mirkin, C. A. Science 2010,
330, 66.
G
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(
8) Magenau, A. J. D.; Strandwitz, N. C.; Gennaro, A.; Matyjaszewski,
K. Science 2011, 332, 81.
9) Lorkovic, I. M.; Duff, R. R.; Wrighton, M. S. J. Am. Chem. Soc.
995, 117, 3617.
10) Allgeier, A. M.; Mirkin, C. A. Angew. Chem., Int. Ed. 1998, 37,
94.
11) Abubekerov, M.; Shepard, S. M.; Diaconescu, P. L. Eur. J. Inorg.
Chem. 2016, 2016, 2634.
12) Wang, X.; Brosmer, J. L.; Thevenon, A.; Diaconescu, P. L.
Organometallics 2015, 34, 4700.
(45) Johnson, C.; Long, B.; Nguyen, J. G.; Day, V. W.; Borovik, A. S.;
Subramaniam, B.; Guzman, J. J. Phys. Chem. C 2008, 112, 12272.
(46) Kochem, A.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.;
Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. Inorg. Chem.
2012, 51, 10557.
(
1
(
8
(
47) Rhodes, B.; Rowling, S.; Tidswell, P.; Woodward, S.; Brown, S.
M. J. Mol. Catal. A: Chem. 1997, 116, 375.
48) Clarke, R. M.; Hazin, K.; Thompson, J. R.; Savard, D.; Prosser,
K. E.; Storr, T. Inorg. Chem. 2016, 55, 762.
49) Abel, E. W.; Long, N. J.; Orrell, K. G.; Osborne, A. G.; Sik, V. J.
Organomet. Chem. 1991, 403, 195.
(
(
(
̌
(
(
(
13) Abubekerov, M.; Diaconescu, P. L. Inorg. Chem. 2015, 54, 1778.
14) Wang, X.; Thevenon, A.; Brosmer, J. L.; Yu, I.; Khan, S. I.;
(
(
00.
50) Sixt, T.; Fiedler, J.; Kaim, W. Inorg. Chem. Commun. 2000, 3, 80.
51) Xue, G.; De Hont, R.; Munck, E.; Que, L. Nat. Chem. 2010, 2,
̈
Mehrkhodavandi, P.; Diaconescu, P. L. J. Am. Chem. Soc. 2014, 136,
1264.
15) Upton, B. M.; Gipson, R. M.; Duhovic, S.; Lydon, B. R.;
4
1
(
(52) Kurahashi, T.; Fujii, H. Inorg. Chem. 2013, 52, 3908.
(53) Waser, J.; Carreira, E. M. J. Am. Chem. Soc. 2004, 126, 5676.
Matsumoto, N. M.; Maynard, H. D.; Diaconescu, P. L. Inorg. Chem.
Front. 2014, 1, 271.
(
J.; Miller, J. T.; Meyer, K.; Cantat, T.; Diaconescu, P. L. Chem.
Commun. 2011, 47, 9897.
(
(54) Zhang, Y.-L.; Ruan, W.-J.; Zhao, X.-J.; Wang, H.-G.; Zhu, Z.-A.
Polyhedron 2003, 22, 1535.
16) Broderick, E. M.; Guo, N.; Wu, T.; Vogel, C. S.; Xu, C.; Sutter,
(55) Dodd, D.; Johnson, M. D. J. Organomet. Chem. 1973, 52, 1.
(56) Chang, C. A.; Huang, C. S.; Tu, C. F. Anal. Chem. 1983, 55,
1
(
390.
57) Gridnev, A. A.; Ittel, S. D.; Fryd, M.; Wayland, B. B.
Organometallics 1993, 12, 4871.
17) Broderick, E. M.; Guo, N.; Vogel, C. S.; Xu, C.; Sutter, J.; Miller,
J. T.; Meyer, K.; Mehrkhodavandi, P.; Diaconescu, P. L. J. Am. Chem.
Soc. 2011, 133, 9278.
(
18) Biernesser, A. B.; Delle Chiaie, K. R.; Curley, J. B.; Byers, J. A.
Angew. Chem., Int. Ed. 2016, 55, 5251.
19) Biernesser, A. B.; Li, B.; Byers, J. A. J. Am. Chem. Soc. 2013, 135,
6553.
20) Sauer, A.; Buffet, J.-C.; Spaniol, T. P.; Nagae, H.; Mashima, K.;
Okuda, J. ChemCatChem 2013, 5, 1088.
21) Zhu, Y.; Romain, C.; Williams, C. K. J. Am. Chem. Soc. 2015,
37, 12179.
22) Thevenon, A.; Garden, J. A.; White, A. J. P.; Williams, C. K.
Inorg. Chem. 2015, 54, 11906.
23) Paul, S.; Romain, C.; Shaw, J.; Williams, C. K. Macromolecules
015, 48, 6047.
24) Romain, C.; Williams, C. K. Angew. Chem., Int. Ed. 2014, 53,
607.
25) Neilson, B. M.; Bielawski, C. W. Chem. Commun. 2013, 49,
453.
(
1
(
(
1
(
(
2
(
1
(
5
(
26) Fors, B. P.; Hawker, C. J. Angew. Chem., Int. Ed. 2012, 51, 8850.
(
27) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem., Int. Ed. 2004,
4
(
3, 4460.
28) Musacchio, A. J.; Nguyen, L. Q.; Beard, G. H.; Knowles, R. R. J.
Am. Chem. Soc. 2014, 136, 12217.
29) Wilger, D. J.; GrandjeanJean-Marc, M.; Lammert, T. R.;
Nicewicz, D. A. Nat. Chem. 2014, 6, 720.
30) Shigehisa, H.; Nishi, E.; Fujisawa, M.; Hiroya, K. Org. Lett. 2013,
5, 5158.
31) Shigehisa, H.; Aoki, T.; Yamaguchi, S.; Shimizu, N.; Hiroya, K. J.
Am. Chem. Soc. 2013, 135, 10306.
32) Shigehisa, H.; Koseki, N.; Shimizu, N.; Fujisawa, M.; Niitsu, M.;
Hiroya, K. J. Am. Chem. Soc. 2014, 136, 13534.
(
(
1
(
(
(
(
(
(
33) Cozzi, P. G. Chem. Soc. Rev. 2004, 33, 410.
34) Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421.
35) Katsuki, T. Coord. Chem. Rev. 1995, 140, 189.
36) Shafir, A.; Fiedler, D.; Arnold, J. J. Chem. Soc., Dalton Trans.
2
002, 555.
(
(
(
37) Broderick, E. M.; Diaconescu, P. L. Inorg. Chem. 2009, 48, 4701.
38) Quan, S. M.; Diaconescu, P. L. Chem. Commun. 2015, 51, 9643.
39) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(
(
(
40) Evans, D. F. J. Chem. Soc. 1959, 2003.
41) Schubert, E. M. J. Chem. Educ. 1992, 69, 62.
42) Leung, W.-H.; Chan, E. Y. Y.; Chow, E. K. F.; Williams, I. D.;
Peng, S.-M. J. Chem. Soc., Dalton Trans. 1996, 1229.
43) Raisanen, M. T.; de Almeida, P.; Meinander, K.; Kemell, M.;
Mutikainen, I.; Leskela, M.; Repo, T. Thin Solid Films 2008, 516, 2948.
44) Yang, L.; Powell, D. R.; Houser, R. P. Dalton Trans. 2007, 955.
(
̈
̈
̈
(
H
DOI: 10.1021/acs.organomet.6b00317
Organometallics XXXX, XXX, XXX−XXX