Catalytic hydrosilylation of ketones using a Co/Zr heterobimetallic complex: Evidence for an unusual mechanism involving ketyl radicals

Article abstract of DOI:10.1021/om301194g

The tris(phosphinoamide)-linked heterobimetallic Co/Zr complex (THF)Zr(MesNPⁱPr₂)₃CoN₂ (1) has been investigated as a catalyst for the hydrosilylation of ketones with PhSiH ₃. Catalytic activity superior to monometallic Co or Zr analogues has been observed, demonstrating the importance of cooperative reactivity between Co and Zr. Upon examining stoichiometric reactions, complex 1 was found to be unreactive toward PhSiH₃, implying that the mechanism diverges from the typical Chalk-Harrod-type hydrosilylation pathway. In contrast, 1 reacts readily with ketones, and in the case of benzophenone, a radical coupling product [(Ph₂CO)Zr(MesNPⁱPr₂) ₃CoN₂]₂ (3) was isolated, implying the intermediacy of a Zr-bound ketyl radical fragment. A radical-based hydrosilylation mechanism is proposed involving hydrogen atom transfer from PhSiH₃ to the Zr-bound ketyl-radical.

Full text of DOI:10.1021/om301194g

Article
pubs.acs.org/Organometallics
Catalytic Hydrosilylation of Ketones Using a Co/Zr Heterobimetallic
Complex: Evidence for an Unusual Mechanism Involving Ketyl
Radicals
Wen Zhou,† Seth L. Marquard,† Mark W. Bezpalko, Bruce M. Foxman, and Christine M. Thomas*
Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts, 02454, United States
S
* Supporting Information
ABSTRACT: The tris(phosphinoamide)-linked heterobimetallic
Co/Zr complex (THF)Zr(MesNPiPr2)3CoN2 (1) has been
investigated as a catalyst for the hydrosilylation of ketones with
PhSiH3. Catalytic activity superior to monometallic Co or Zr
analogues has been observed, demonstrating the importance of
cooperative reactivity between Co and Zr. Upon examining
stoichiometric reactions, complex 1 was found to be unreactive
toward PhSiH3, implying that the mechanism diverges from the
typical Chalk−Harrod-type hydrosilylation pathway. In contrast, 1
reacts readily with ketones, and in the case of benzophenone, a
radical coupling product [(Ph2CO)Zr(MesNPiPr2)3CoN2]2 (3) was
isolated, implying the intermediacy of a Zr-bound ketyl radical
fragment. A radical-based hydrosilylation mechanism is proposed
involving hydrogen atom transfer from PhSiH3 to the Zr-bound
ketyl-radical.
benzaldehyde, but little mechanistic insight was provided and
catalytic activities were similar to those observed with
monometallic Rh complexes.36 Similarly, Comte and Le
Gendre et al. reported that ketone hydrosilylation with a
heterobimetallic Ti−Rh complex proceeds more rapidly than
with similar monometallic Rh or Ti complexes.37 Notably, these
heterobimetallic examples of ketone hydrosilylation catalysts
utilize metals that are both known to be catalytically competent
as monometallic species. A more interesting target, however, is
a heterobimetallic system that promotes reactivity that is
inaccessible with a monometallic analogue.
Our own work on heterobimetallic complexes focuses on
tris(phosphinoamide)-linked early/late heterobimetallic combi-
nations, most notably Zr/Co complexes.38 We previously
reported the synthesis and characterization of a highly reduced
Zr/Co complex, (THF)Zr(MesNPiPr2)3CoN2 (1, Mes = 2,4,6-
trimethylphenyl), featuring metal−metal multiple bonds and
the weakly coordinating labile apical ligands THF and N2.39
The reduced complex 1 has been shown to react with a series of
small molecule substrates, such as I2,40 CO2,41 H2,40
hydrazines,42 and alkyl halides.40 The reactions with RX led
us to investigate the catalytic activity of heterobimetallic Zr/Co
complexes toward Kumada-type cross-coupling reactions
between alkyl halides and alkyl Grignard reagents, revealing
enhanced catalytic activity relative to monometallic Zr or Co
analogues.43
INTRODUCTION
■
Hydrosilylation is a fundamental organometallic transformation
for the reduction of alkenes, carbonyl, and imine function-
alities.1 The most common ketone hydrosilylation catalysts
feature noble metals, such as Rh2 and Ir;3 however, hydro-
silylation is now well-precedented for a variety of first-row
transition metals, including titanium,4−8 zinc,9,10 copper,6,11−14
nickel,15−18 and iron.19−23 In contrast, there are no reported
zirconium-based hydrosilylation catalysts and cobalt-based
carbonyl hydrosilylation catalysts have received limited
attention.24−27 Most ketone hydrosilylation mechanisms invoke
metal hydride intermediates in the catalytic cycle, but the
mechanism of action of cobalt and iron catalysts is not well-
understood.22 Given the propensity of first-row transition
metals to undergo one-electron redox processes, one might
predict alternative radical hydrosilylation mechanisms to be
competitive (if not operative) pathways; however, no firm
evidence for such routes has been presented.
A recent theme emerging in catalytic methodology is the use
of bimetallic metal complexes to catalyze organic trans-
formations.28−30 While studies of homobimetallic catalysts are
now quite common, comparatively little attention has been
placed on early/late heterobimetallic catalysts,31 particularly
given the vast literature on the synthesis of early/late
heterobimetallic complexes.32−35 Several researchers have
investigated ketone hydrosilylation reactions catalyzed by
early/late heterobimetallic complexes.31 Nikonov and co-
workers screened several heterobimetallic Nb−Rh complexes
for the catalytic hydrosilylation of acetophenone and
Received: December 10, 2012
Published: February 22, 2013
© 2013 American Chemical Society
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We postulated that reduction of polar substrates, such as
ketones, would also be a promising application for Zr/Co
heterobimetallic complexes, owing to the polar nature of the
Zr−Co bonds in our catalyst precursors and the oxophilic
nature of Zr. We began by first examining the hydrosilylation of
ketones, and herein we describe the catalytic activity of complex
1 toward the hydrosilylation of ketones and α,β-unsaturated
enones, and provide evidence for an unusual radical hydro-
silylation mechanism.
with no evidence for reduction of the alkene (entry 5).
Complex 1 was also active toward more sterically hindered aryl-
substituted ketones (entries 6−8), although in these cases,
significantly longer reaction times were required to achieve a
high yield at room temperature. Particularly bulky and/or
electron-rich ketones (entries 9−13) required long reaction
times at elevated temperature (65 °C), and in some cases, 20
mol % LiI was required to obtain moderate-to-high yields.
Other additives and solvents were screened in an effort to
optimize these reactions, but LiI proved to be the most effective
additive (see the Supporting Information). While 5 mol % of
complex 1 was utilized for all the reactions shown in Table 1,
lower catalyst loadings (2.5 mol %) were also effective, albeit
with longer reaction times (Table 2, entries 1−3).
Examination of the silylated intermediates by GC/MS prior
to acidic workup revealed a ∼3:1 ratio of the mono- and
disubstituted silylether products from the crude hydrosilylation
reaction with a 1:1 silane/ketone ratio (PhH2SiOCHMePh and
PhHSi(OCHMePh)2 in the case of acetophenone; see the
Supporting Information). Subsequent hydrolysis of either of
these silylethers via addition of 4 M HCl afforded the free
alcohol product. Thus, in some cases, a substoichiometric
amount of PhSiH3 could be used with no effect on product
yield (Table 1, entry 1). Other silanes, such as Ph3SiH,
iPr2ClSiH, and Et2SiH2, were also investigated; however, the
yields and reaction times were far inferior to those with PhSiH3
(see the Supporting Information).
RESULTS AND DISCUSSION
■
In a simple test reaction, we found that acetophenone was
cleanly hydrosilylated by PhSiH3 in the presence of 5 mol % of
complex 1 after 6 h at room temperature to afford the
corresponding alcohol after acidic workup (Table 1, entry 1). A
number of alkyl and aryl ketones were screened to investigate
the substrate scope of this catalytic reaction. Dialkyl ketones
were hydrosilylated in very high yields at room temperature
(entries 2−4), and the ketone functionality of the unsaturated
ketone 5-hexene-2-one was hydrosilylated in high yield (98%)
Table 1. Hydrosilylation of Ketones with Phenylsilane
a b c d
, , ,
Catalyzed by Complex 1
To ascertain the role of Co and Zr and the potential
cooperative reactivity between these two metals, monometallic
analogues of complex 1 were also screened as ketone
hydrosilylation catalysts (Table 2). The monometallic Zr
metalloligand precursor (iPr2PNMes)3ZrCl did not show any
hydrosilylation activity with benzophenone (entry 4), nor did a
combination of CoI2 with 3 equiv of phosphinoamine ligand to
generate a monometallic Co complex in situ (entry 5). The
isolated Co tris(phosphinoamine) complex ICo(Ph2PNHiPr)3
did afford the hydrosilylation product, but in low yield (30%,
entry 6). Similar diminished yields were observed, even with
longer reaction times, when other CoI tris(phosphine) catalysts,
such as (PPh3)3CoI, were employed as a catalyst (entries 7 and
8). These important control reactions imply that both Co and
Zr are playing a role in the catalytic hydrosilylation reaction.
Notably, the CO-ligated derivative of 1, (THF)Zr(MesN-
PiPr2)3Co(CO), was far less active than 1 (entry 9). At this
point, we are unable to discern whether this is a consequence of
the less labile CO ligand bound to Co or the decreased
reducing nature of this complex.
Several α,β-unsaturated enones were also investigated under
our standard hydrosilylation reaction conditions (Table 3).
Complex 1 served as an effective catalyst for conjugate
hydrosilylation; however, relatively long reaction times (24 h)
and elevated temperatures (65 °C) were required. Chalcone
(entry 1) and 1-phenylpent-2-en-1-one (entry 2) both
underwent conjugate reduction readily under these conditions
to afford moderate-to-high yields of the corresponding ketone
upon acid hydrolysis of the silyl enol ether product. No
evidence for the 1,2-reduction products (alcohols) was
observed by GC-MS. Reduction of benzylideneacetone,
however, proceeded more sluggishly and required the addition
of 20 mol % LiI, and a mixture of the ketone and unsaturated
alcohol product was isolated. In contrast, the hydrosilylation of
an α,β-unsaturated aldehyde (entry 4) afforded the unsaturated
alcohol product exclusively in high yield.
a
b
Yield determined using GC/MS. Isolated yield after column
c
chromatography. 20 mol % LiI was added to these reactions to
improve the yield. This reaction was run with a ketone/silane ratio of
1.4:1, showing that 1 equiv of silane can serve to reduce more than 1
equiv of ketone.
d
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Table 2. Comparison of Catalytic Hydrosilylation Activity of Complex 1 to Monometallic Co and Zr Complexes
entry
catalyst
catalyst loading (mol %)
time (h)
yield (%)
b
a
a
a
a
b
b
b
b
1
2
3
4
5
6
7
8
9
1
1
1
5
12
12
18
12
12
12
12
18
12
98
61
92
0
2.5
2.5
5
(iPr2PNMes)3ZrCl
CoI2 + MesNHPiPr2
ICo(Ph2PNHiPr)3
ICo(PPh3)3
5/15
5
0
30
45
63
40
5
ICo(PPh3)3
(THF)Zr(MesNPiPr2)3Co(CO)
5
5
a
b
Yield determined using GC/MS. Isolated yield after column chromatography.
Table 3. Reduction of α,β-Unsaturated Enones with
Phenylsilane Catalyzed by Complex 1
Scheme 1
ab c d e
, , , ,
a
b
Isolated yield after column chromatography. Ratio between the two
c
products of entry 3 is 2.6:1 by GC/MS. Yield determined using GC/
MS. The reaction was conducted at room temperature for 6 h. 20
mol % LiI was necessary to achieve reasonable yields.
d
e
To gain some insight into the mechanism of this hydro-
silylation reaction, stoichiometric reactions were conducted. On
the basis of the canonical Chalk−Harrod hydrosilylation
mechanism,44 one might have envisioned a mechanism
whereby silane is first oxidatively added to the metal center(s),
followed by insertion of the ketone substrate into a M−H or
M−Si bond.1 Unexpectedly, however, no reaction occurs when
complex 1 is treated with PhSiH3 at room temperature based
1
on H NMR spectroscopy (Scheme 1).
Since complex 1 does not react with PhSiH3, we investigated
the reaction between 1 and a typical ketone substrate,
benzophenone. Interestingly, this reaction resulted in the
formation of a new complex 3 with a complex broad,
1
paramagnetically shifted H NMR spectrum (Scheme 1 and
Figure 1. Displacement ellipsoid (50%) representation of complex 3.
Hydrogen atoms except for those bound to C54 have been omitted for
clarity. Selected bond lengths (Å): Co−Zr, 2.744(3); Zr−O1,
1.9785(15); O1−C47, 1.477(4); C47−C54, 1.496(5); C54−C55,
1.478(3); C55−C56, 1.343(3); C46−C54, 1.364(4); C46−O1,
1.392(4).
Supporting Information). The solid-state structure of 3 was
determined by single-crystal X-ray diffraction (Figure 1),
revealing that radical coupling of two ketyl radical fragments
has occurred to generate an isobenzopinacol fragment, resulting
in an asymmetric tetrametallic complex. The C−C bond
lengths corresponding to the coupled benzophenone units in
complex 3 are reminiscent of a formulation similar to
Gomberg’s dimer,45−48 with lengthened and contracted C−C
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distances in line with the loss of aromaticity in the central six-
membered ring (Figure 1).
Scheme 2
The benzophenone radical coupling product 3 is similar to
the radical coupling products that have been reported using
uranium(III)49,50 and titanium(III)51−53 complexes. Similar to
previous reports, we propose that complex 3 forms via the
benzophenone ketyl radical intermediate 4 (Scheme 1) and
steric factors likely preclude the formation of the more
traditional pinacol coupling products.54−56 A computational
(DFT) investigation of ketyl radical complex 4 was performed
using Gaussian 09,57 and a resulting plot of the Mulliken
unpaired spin density is shown in Figure 2. In this triplet
the initial reaction rates for the hydrosilylation reaction of
benzophenone with PhSiH3 and PhSiD3 revealed an estimated
kinetic isotope effect (KIE) kH/kD = 2.1, suggesting that Si−H
bond cleavage is rate-determining (see the Supporting
Information).
On the basis of the stoichiometric reactions and the
deuterium labeling experiments described above, a catalytic
cycle for the hydrosilylation reaction is proposed (Scheme 3).
Scheme 3
Figure 2. Plot of the Mulliken unpaired spin density surface of the
purported ketyl radical complex 4 as calculated using DFT methods
(BP86/LANL2TZ(f)/6-311G(d,p)/D95).
species, one unpaired electron resides on the Co center
(formally, Co0), while the second unpaired electron is
delocalized throughout the Zr and its bound benzophenone
ketyl ligand. While a small percentage of the unpaired spin
resides on the Zr center (0.15 electrons), most of the spin
density resides on the carbonyl carbon (0.60 electrons) and the
ortho and para (0.18 and 0.20 electrons, respectively) carbons
of one of the aromatic rings of the benzophenone.
Intermolecular coupling of this ketyl radical through the para
and benzylic carbons is consistent with this computational
prediction. The asymmetric benzophenone dimer 3 does react
with PhSiH3 at room temperature over a period of several
hours to form the expected silylether product (PhSiH2OCH-
Ph2), indicating that the radical coupling process is reversible,
consistent with the weak C−C bonds in Gomberg’s dimer
(11−12 kcal/mol)47 and in [(silox)3TiIII(OCPh2)]2 (18 kcal/
mol).51 Consistent with the specificity of the catalytic process
to primary silanes, Ph3SiH does not react with complex 3 under
ambient conditions.
Deuterium labeling studies were carried out to further probe
the hydrosilylation mechanism. As would be expected in a
prototypical hydrosilylation reaction, treatment of methylnap-
thylketone with phenylsilane-d3 under catalytic conditions (5
mol % 1, 20 mol % LiI, 65 °C, 24 h) resulted exclusively in the
product with a deuterium atom in the benzylic position after
acidic workup (Scheme 2). Hydrosilylation of the α,β-
unsaturated ketone chalcone also undergoes selective deute-
rium incorporation, this time at the β-position. Comparison of
Unlike the typical Chalk−Harrod hydrosilylation mechanism,
the first step in this Zr/Co-catalyzed process involves ketone
coordination, rather than Si−H activation. Upon coordination,
an electron is transferred to the ketone, generating a ketyl
radical intermediate. Similar one-electron transfer reactions to
substrates bound through Zr have been proposed previously
with these Zr/Co heterobimetallic systems,42,58 and this one-
electron transfer process is proposed to occur via homolytic
cleavage of the Co−Zr bond. The ketyl radical intermediate can
dimerize, as in the case of complex 3, or can be intercepted as a
monomer by a PhSiH3. We suggest that the silane reacts by
donating a hydrogen atom to the bound ketyl radical,59 with
concomitant formation of a Si−O bond and liberation of the
resulting silyl ether to regenerate catalyst 1. This is consistent
with the relatively large KIE of 2.1, which implies that H-atom
abstraction is rate-determining. Since this is an inner-sphere
process, we propose that the bulky mesityl amide substituents
preclude approach of the silane when di- or trisubstituted
silanes are used, resulting in no hydrosilylation reactivity when
silane reagents bulkier than PhSiH3 are used. The isolated
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[(Ph2CO)Zr(MesNPiPr2)3CoN2]2 (3). In a 20 mL scintillation vial,
(THF)Zr(MesNPiPr2)3CoN2 (80 mg, 0.080 mmol) was dissolved in 2
mL of diethyl ether. To this solution was added a solution of
benzophenone (16 mg, 0.086 mmol) in 2 mL of diethyl ether. After 5
min of swirling at room temperature, the yellow-orange solution was
allowed to stand at −35 °C for 12 h. The brownish-yellow supernatant
was decanted from the yellow crystalline solid and discarded. The
yellow solid was dried in vacuo, affording 76 mg (85%) of complex 3.
1H NMR (400 MHz, C6D6): δ 9.09, 88.84, 8.59, 8.37, 8.07, 7.99, 7.89,
7.77, 7.68 (m), 7.56 (m), 7.46, 7.36, 7.27, 7.10, 7.00, 6.93, 6.49, 6.34,
5.83, 5.48, 5.41, 5.14, 4.87, 4.04, 3.98, 3.10, 3.00, 2.91, 2.39, 2.31, 2.16,
2.12, 1.98, 1.93, 0.57, 0.13, −0.54, −2.27, −2.75, −3.62, −3.86. UV−
vis (C6H6, λ(nm) (ε, M−1 cm−1): 331 (2.2 × 104). Evans’ method
(298 K, C6D6): 2.41 μB. IR (C6H6): 2046 cm−1 (Co-N2), 1776 cm−1
(C-O). Because of the lability of the N2 ligand, satisfactory elemental
analysis was conducted on the N2-free complex. Anal. Calcd for
C116H170N6O2P6Co2Zr2: C, 64.30; H, 7.91; N, 3.88. Found: C, 64.22;
H, 7.98; N, 3.82.
Typical Procedure for Hydrosilylation of Ketones. In a
nitrogen-filled glovebox, benzophenone (45.0 mg, 0.247 mmol) and
phenylsilane (26.8 mg, 0.247 mmol) were weighed into a 20 mL vial.
Dry THF (5 mL) and complex 1 (11.5 mg, 0.0115 mmol, 5 mol %)
were added. The mixture was stirred at ambient temperature for 12 h.
The reaction was removed from the glovebox, and 4 M aqueous HCl
solution (1.5 mL) was added to quench the reaction. The organic layer
was dried with MgSO4 and filtered, and solvent was subsequently
removed in vacuo. The remaining residue was purified by silica gel
column chromatography (10:1 hexanes/ethyl acetate). Yield: 44.4 mg
(97.6%).
Typical Procedure for Hydrosilylation of α,β-Unsaturated
Enones. In a nitrogen-filled glovebox, (E)-chalcone (140 mg, 0.69
mmol) and phenylsilane (90 mg, 0.83 mmol) were weighed into a 20
mL vial. Dry THF (7 mL) and complex 1 (32 mg, 0.032 mmol, 5 mol
%) were added, and the mixture was stirred at 65 °C for 24 h. The
reaction was removed from the glovebox, and 4 M HCl aqueous
solution (1.5 mL) was added to quench the reaction. The organic layer
was dried with MgSO4 and filtered, and the solvent was subsequently
removed in vacuo. The remaining residue was purified by silica gel
column chromatography (10:1 hexanes/ethyl cetate). Yield: 131 mg
(90.0%).
dimerized ketyl radical complex 3 (2.5 mol %) was shown to be
a competent catalyst for the hydrosilylation of isopropyl phenyl
ketone, affording the corresponding alcohol product in 99%
isolated yield upon acid hydrolysis.
These results provide compelling evidence for the
intermediacy of the ketyl radical complex 4 in the catalytic
hydrosilylation mechanism of benzophenone. We recognize,
however, that a different mechanism may be operative for
ketones without an aromatic ring to stabilize a ketyl radical
intermediate via delocalization. We also note that the
requirement of LiI as an additive in certain cases (Table 1,
entries 10−13) points toward a more canonical polar-type
mechanism in the case of these particularly challenging
substrates. However, a control reaction in which 1 equiv of
LiI was added to benzophenone dimer 3 lead to formation of
an as-yet-unidentified new product, implicating that a simple
“Lewis acid” explanation for the role of LiI may not be valid.
CONCLUSION
■
In conclusion, we have examined the catalytic hydrosilylation of
ketones by a Zr/Co heterobimetallic complex and have
uncovered potential catalytically relevant intermediates that
suggest a radical mechanism for this process. The Zr/Co
heterobimetallic shows catalytic activity superior to that of Zr
or Co monometallic analogues, implying that both Co and Zr
play an important role in the catalytic process. It appears, based
on stoichiometric reactions, that the catalytic process occurs at
the Zr center and Co simply serves to facilitate radical reactivity
at Zr by stabilizing the active ZrIII center via metal−metal
bonding. It is important to point out that this catalyst features
inexpensive and naturally abundant metals, and synthetically
straightforward ligands. Future studies will focus on developing
this and other reactions catalyzed by 1, and further investigation
of the mechanism and the unusual ketyl radical intermediates.
EXPERIMENTAL SECTION
PhSiD3. This reagent was prepared following a modification of the
literature procedure,62 as follows. Solid LiAlD4 (43 mg, 1 mmol) was
added slowly to a solution of trichlorophenylsilane (210 mg, 1 mmol)
in THF (7 mL) at −30 °C. The reaction was stirred for 16 h. The
resulting solution was filtered through Celite to remove Li salts. The
solvent was removed from the filtrate carefully at −30 °C to remove
most of the Et2O. The resulting PhSiD3 was used without further
purification.
■
General Considerations. All manipulations were carried out
under an inert atmosphere using a nitrogen-filled glovebox or standard
Schlenk techniques unless otherwise noted. All glassware was oven or
flame-dried immediately prior to use. Tetrahydrofuran and diethyl
ether were obtained as HPLC grade without inhibitors; pentane, and
benzene were obtained as ACS reagent grade. Tetrahydrofuran, diethyl
ether, and toluene were degassed and dried by sparging with ultra-
high-purity argon gas, followed by passage through a series of drying
columns using a Seca Solvent System by Glass Contour. All solvents
were stored over 3-Å molecular sieves. Solvents were frequently tested
using a standard solution of sodium benzophenone ketyl in
tetrahydrofuran to confirm the absence of oxygen and moisture.
Representative Procedure for NMR Kinetics Used To
Estimate KIE. A screw-top NMR tube was charged with
trimethoxybenzene (1.1 mg, 0.0065 mmol, internal standard) and
PhSiH3 (11.2 μL, 0.0910 mmol). In a 20 mL vial, complex 1 (4.6 mg,
0.0046 mmol) was dissolved in 0.7 mL of THF-d8. The solution of 1
was added to a separate 20 mL vial containing benzophenone (16.8
mg, 0.0920 mmol). The resulting yellow solution was transferred to
the NMR tube and mixed by shaking. The reaction progress was
́
Benzene-d6 was degassed and dried over 4 Å molecular sieves before
use. Complex 1 ((THF)Zr(MesNPiPr2)3CoN2),39 ICo(Ph2P-
NHiPr)3,60 (THF)Zr(MesNPiPr2)3Co(CO),58 and (E)-1-phenylpent-
2-en-1-one61 were prepared following literature procedures. All other
reagents were purchased from Aldrich, Acros Organics, or Alfa Aesar
and dried and degassed thoroughly prior to use. NMR spectra were
recorded at ambient temperature unless otherwise stated on a Varian
1
monitored by H NMR spectroscopy at 25 °C taking scans every 5
min. The disappearance of the silane was recorded relative to the
internal standard, and the data were processed using KaliedaGraph
software.
1
Inova 400 MHz instrument. H and 13C NMR chemical shifts were
X-ray Crystallography. All operations were performed on a
Bruker-Nonius Kappa Apex2 diffractometer, using graphite-mono-
chromated Mo Kα radiation. All diffractometer manipulations,
including data collection, integration, scaling, and absorption
corrections, were carried out using the Bruker Apex2 software.63
Preliminary cell constants were obtained from three sets of 12 frames.
Additional crystallographic details and final CIF data are included as
Supporting Information.
referenced to residual solvent and are reported in parts per million.
GC-MS data were collected on an Agilent 7890A GC System and
5975C VL MSD with a Triple-Axis Detector, and yields were reported
via integration of starting materials and products. Infrared spectra were
recorded on a Varian 640-IR spectrometer controlled by Resolutions
Pro software. UV−vis spectra were recorded on a Cary 50 UV−vis
spectrophotometer using Cary WinUV software. Elemental micro-
analyses were performed by Complete Analysis Laboratories, Inc.,
Parsippany, NJ.
Computational Details. All calculations were performed using
Gaussian 09, Revision A.02, for the Linux operating system.57 Density
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functional theory calculations were carried out using a combination of
Becke’s 1988 gradient-corrected exchange functional64 and Perdew’s
1986 electron correlation functional65 (BP86). A mixed-basis set was
employed, using the LANL2TZ(f) triple-ζ basis set with effective core
potentials for cobalt and zirconium,66 Gaussian 09’s internal 6-
311+G(d) for heteroatoms (nitrogen, oxygen, phosphorus), and
Gaussian 09’s internal LANL2DZ basis set (equivalent to D95 V67) for
carbon and hydrogen. Using crystallographically determined geo-
metries as a starting point, the geometries were optimized to a
minimum, followed by analytical frequency calculations to confirm that
no imaginary frequencies were present.
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ASSOCIATED CONTENT
* Supporting Information
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S
Additional spectral data, screening of different reaction
conditions, computational and crystallographic materials, and
crystallographic data for 3 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Author Contributions
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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The authors acknowledge the U.S. Department of Energy
(under Award No. DE-SC0004019) and Brandeis University
for funding this work. C.M.T. is also grateful for a 2011 Alfred
P. Sloan Fellowship. The authors also thank Jeremy P.
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