Reactivity of Cr(III) μ-oxo compounds: Catalyst regeneration and atom transfer processes

Article abstract of DOI:10.1021/ic202233f

Oxidation of CpCr[(XylNCMe)₂CH] (Xyl = 2,6-Me₂C ₆H₃) with pyridine N-oxide or air generated the μ-oxo dimer, {CpCr[(XylNCMe)₂CH]}₂(μ-O). The μ-oxo dimer was converted to paramagnetic Cr(III

Full text of DOI:10.1021/ic202233f

Article
pubs.acs.org/IC
Reactivity of Cr(III) μ-Oxo Compounds: Catalyst Regeneration and
Atom Transfer Processes
K. Cory MacLeod,† Brian O. Patrick,‡ and Kevin M. Smith*,†
†Department of Chemistry, University of British Columbia Okanagan, 3333 University Way, Kelowna, BC, Canada V1V 1V7
‡Department of Chemistry, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1
S
* Supporting Information
ABSTRACT: Oxidation of CpCr[(XylNCMe)2CH] (Xyl =
2,6-Me2C6H3) with pyridine N-oxide or air generated the μ-
oxo dimer, {CpCr[(XylNCMe)2CH]}2(μ-O). The μ-oxo
dimer was converted to paramagnetic Cr(III) CpCr-
[(XylNCMe)2CH](X) complexes (X = OH, O2CPh, Cl,
OTs) via protonolysis reactions. The related Cr(III) alkoxide
complexes (X = OCMe3, OCMe2Ph) were prepared by salt
metathesis and characterized by single crystal X-ray diffraction.
The interconversion of the Cr(III) complexes and their
reduction back to Cr(II) with Mn powder were monitored
using UV−vis spectroscopy. The related CpCr-
[(DepNCMe)2CH] (Dep = 2,6-Et2C6H3) Cr(II) complex
was studied for catalytic oxygen atom transfer reactions with PPh3 using O2 or air. Both Cr(II) complexes reacted with pyridine
N-oxide and γ-terpinene to give the corresponding Cr(III) hydroxide complexes. When CpCr[(DepNCMe)2CH] was treated
with pyridine N-oxide in benzene in the absence of hydrogen atom donors, a dimeric Cr(III) hydroxide product was isolated and
structurally characterized, apparently resulting from intramolecular hydrogen atom abstraction of a secondary benzylic ligand C−
H bond followed by intermolecular C−C bond formation. The use of very bulky hexaisopropylterphenyl ligand substituents did
not preclude the formation of the analogous μ-oxo dimer, which was characterized by X-ray diffraction. Attempts to develop a
chromium-catalyzed intermolecular hydrogen atom transfer process based on these reactions were unsuccessful. The protonolysis
and reduction reactions of the μ-oxo dimer were used to improve the previously reported Cr-catalyzed radical cyclization of a
bromoacetal.
INTRODUCTION
■
The use of first-row transition metals is becoming more
common in catalytic C−C bond forming reactions for organic
synthesis.1 Compared to their heavier congeners, first-row
metal complexes are cheaper and often exhibit complementary
reactivity, but they also have a recognized propensity to engage
in radical reactivity.2 Redox noninnocent ligands have been
apparent decomposition, and even CpCr[(XylNCMe)2CH](R)
employed to enforce two-electron chemistry.3 Alternatively,
alkyl complexes can be remarkably resistant to O2 if protected
from ambient light.8 However, similar to most reduced metal
applications have been developed that explicitly harness the
metal center’s ability to engage in single-electron transfer
complexes capable of single-electron oxidative addition of
relatively strong carbon−halogen bonds,9,10 these Cr(II)
compounds are highly air-sensitive. The CpCr-
[(XylNCMe)2CH] system would be more generally applicable
if its tolerance to air and other impurities in substrates and
solvents could be improved.
reactivity.4 We have been using well-defined cyclopentadienyl
chromium β-diketiminate complexes to investigate the
reactivity of Cr(II) and Cr(III) complexes.5 Oxidative addition
of organic halides, RX, with the Cr(II) compound CpCr-
[(XylNCMe)2CH] (1) gives a 1:1 mixture of the Cr(III) halide
and Cr(III) alkyl complexes.6 Use of excess RX and PbX2-
activated manganese as a stoichiometric reductant of CpCr-
[(XylNCMe)2CH](X), where X = Br or Cl (2) provides a
convenient synthetic route to CpCr[(XylNCMe)2CH](R)
alkyls. We have applied this reactivity to the catalytic radical
cyclization of haloacetals (eq 1).7
In this paper, we examine the problem of air-sensitivity in the
radical reactivity of CpCr[(ArNCMe)2CH] using stoichiomet-
ric manganese as a reductant. The product of air oxidation of
CpCr[(XylNCMe)2CH], 1, is the Cr(III) μ-oxo complex
3.11−13 Although both the Cp and β-diketiminate ligands
The paramagnetic Cr(III) halide complexes are air-stable as
crystalline solids that can be stored in air for years without any
Received: October 14, 2011
Published: December 16, 2011
© 2011 American Chemical Society
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remain intact and Cr-bound in 3, the Cr(III)−O bonds were
expected to pose a significant obstacle to reforming the reactive
Cr(II) complex, 1 (Scheme 1). We report the conversion of the
Scheme 1
1
Figure 1. Thermal ellipsoid diagram (50%) of 3.
/
Molecule of
2
hexanes and all H atoms are omitted for clarity.
respect to the other. The staggered geometry of the ligands is
presumably a result of steric strain in the molecule due to the
xylyl ortho-methyl groups of the β-diketiminate ligands, which
also exhibit elongation of the Cr−N bonds (2.046−2.055 Å), a
trend previously observed with only the most sterically
encumbered trivalent CpCr[(ArNCMe)2CH](R) alkyl com-
pounds.8
Air exposure of a solution of Cr(II) complex 1 resulted in a
rapid color change from green to orange, with a UV−vis
spectrum that matched that of the μ-oxo compound 3.
Attempts to determine the rate of reaction of O2 with
compound 1 were unsuccessful, as the reaction of a 9.8 ×
10−5 M solution of 1 with 10 equiv of dry O2 went to
completion shortly after mixing (<5 s). Previous work by Liston
and West reported the formation of a Cr(III) μ-oxo porphyrin
species from oxidation of Cr(II) porphyrins with oxygen
(Scheme 2).15 They proposed that the first step involved the
μ-oxo product to other Cr(III) species more amenable to
reduction to Cr(II) under catalytically relevant conditions,
including the independent synthesis, spectroscopic character-
ization, and structural elucidation of these Cr(III) species. The
scope of manganese reactivity with Cr(III) species was
examined as well as catalyst regeneration of the μ-oxo complex
3 for haloacetal cyclization. The β-diketiminate ligand was
modified to discourage μ-oxo formation, leading to examination
of the reactivity of a putative Cr(IV) oxo intermediate. Catalytic
conditions for PPh3 oxidation by oxygen atom transfer with O2
or air as oxygen atom sources, and intra- and intermolecular
hydrogen atom transfer (HAT) reactivity is also presented.
Scheme 2
RESULTS AND DISCUSSION
Synthesis of {CpCr[(XylNCMe)2CH]}2(μ-O). The Cr(III)
μ-oxo complex {CpCr[(XylNCMe)2CH]}2(μ-O), 3, was
■
formation of a Cr(III) superoxo species that was subsequently
trapped by another equiv of Cr(II) porphyrin. This
intermediate then undergoes O−O bond cleavage forming
two Cr(IV) oxo porphyrin compounds, which were found to
react reversibly in solution with the Cr(II) porphyrin starting
material to form a Cr(III) μ-oxo porphyrin complex. Veige and
co-workers also reported the formation of a Cr(IV) μ-oxo
compound by a Cr(III)/Cr(V) redox couple analogous to the
Cr(II)/Cr(IV) couple proposed in Scheme 2.11f
The reaction of the Cr(II) compound 1 with pyridine N-
oxide under air-free conditions provides a cleaner route for the
isolation of μ-oxo compound 3. Although compound 3 was the
product of air oxidation of 1, a subsequent color change from
orange to green was observed over several hours when
solutions of 3 were subjected to air exposure.
1
prepared by reaction of the Cr(II) complex 1 with /2 equiv
of pyridine N-oxide under an inert atmosphere in hexanes.
Upon stirring overnight, the precipitate was isolated and dried
to provide 3 as an analytically pure orange powder in high yield.
Complex 3 exhibits a strong absorbance peak in the UV−vis
spectrum with λmax = 362 nm (ε = 20700 M−1 cm−1) and a
shoulder at 471 nm. The solution magnetic moment of 2.39 μB
for compound 3 is significantly lower than expected for two
high-spin Cr(III) metal centers. Similar antiferromagnetic
coupling was observed for the Cr(III) μ-oxo compound
[{Cr(NCS)(TPyEA)}2O] (μeff = 1.63 μB/Cr atom),11a as
well as the chromium porphyrin heterobimetallic compounds
(py)(TPP)CrOFe(tmtaa) and (Tpp)CrOFe(Pc) (μeff = 3.51
μB/molecule and 3.12 μB/molecule, respectively).14
Reactivity of {CpCr[(XylNCMe)2CH]}2(μ-O). Attempts to
further oxidize the Cr(III) μ-oxo compound 3 to a higher
valence Cr oxo species were not successful. The reaction of the
Cr(II) complex 1 with an excess of pyridine N-oxide did not
affect the formation of 3 as the sole isolable product (Scheme 3,
path A).16 In addition, 3 was unreactive toward a >10-fold
excess of pyridine N-oxide in a variety of solvents under an
inert atmosphere over a two day period.
Crystallographic analysis of compound 3 confirmed the
presence of the μ-oxo core (Figure 1), with a Cr−O−Cr angle
of 174.5(2)° and Cr−O bond lengths of 1.834(3) Å, similar to
the previously reported [{Cr(NCS)(TPyEA)}2O] (Cr−O−Cr
= 176.5(6)° and Cr−O bond lengths of 1.82(1) and 1.81(1)
Å.11a The solid-state molecular structure exhibits a staggered
type of arrangement for the Cp and β-diketiminate ligands,
1
where /2 of the molecule is rotated approximately 143° with
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Scheme 3
Scheme 4
Figure 2. Thermal ellipsoid diagrams (50%) of (a) 5 and (b) 5a. All H atoms are omitted for clarity.
We found that atmospheric moisture was responsible for the
air-sensitivity of complex 3. Under an inert atmosphere,
complex 3 reacted with a stoichiometric amount of degassed
water to affect the color change previously observed for the
prolonged air exposure of 3 (Scheme 3, path B). The UV−vis
spectrum exhibits a strong absorption at 390 nm and two
weaker absorptions at 506 and 611 nm. Characterization of this
green species confirmed that the water had protonated the μ-
oxo ligand of 3 to produce 2 equiv of a Cr(III) hydroxide
compound CpCr[(XylNCMe)2CH](OH), 4.
The μ-oxo complex 3 was found to react cleanly with a
variety of proton sources HX, where HX = [HNEt3]Cl,
[HLut]Br, [HCol]OTs, PhCO2H (Lut = lutidine, 2,6-
dimethylpyridine; Col = collidine, 2,4,6-trimethylpyridine),
generating the monometallic Cr(III) complexes shown in
Scheme 3, paths C and D. The addition of 2 equiv of acid in
path D cleanly generated 2 equiv of the corresponding
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Cr(III)−X complex, where X = Cl (2), Br, or OTs.
Additionally, the reaction of 3 with 2 equiv of benzoic acid
yielded the Cr(III) benzoate complex CpCr[(XylNCMe)2CH]-
(O2CPh), 5, in 66% isolated yield (Scheme 3, path C).
The clean conversion of the μ-oxo complex 3 to the Cr(III)
halide and benzoate complexes prompted further investigation
(Scheme 3, paths C and D). Reacting isolated Cr(III)
hydroxide 4 with the HX sources under the same reaction
conditions as complex 3 led to the clean formation of the
Cr(III)−X compounds (Scheme 3, paths E and F). This result
suggests that the reaction of 3 with HX proceeds by an initial
reaction of 3 with 1 equiv of HX to form the Cr(III)−X
product and a stoichiometric amount of the hydroxide species
4, which then reacts with a second equiv of HX to produce a
second equiv of Cr(III)−X and 1 equiv of water (Scheme 4).
Notably, isolation of the Cr(III)−X compounds did not appear
to be hindered by the stoichiometric amount of water
presumably generated as the byproduct of the reaction.
Independent Synthesis and Characterization of CpCr-
[(ArNCMe)2CH](X). The benzoate complex 5 was independ-
ently synthesized in good yield by reaction of the Cr(III)
chloride complex 2 with silver benzoate (Scheme 2, path H).
Compound 5 has a strong absorption in the UV−vis spectrum
at 413 nm and two weaker absorptions at 503 and 584 nm and
was structurally characterized by X-ray crystallography (Figure
2a).
In addition to preparing the benzoate complex 5 by salt
metathesis from the chloride precursor 2, we previously
reported the synthesis of the acetate analogue CpCr-
[(XylNCMe)2CH](O2CMe) from compound 2 and
AgO2CMe.17 A Cr(III) benzoate species was also prepared
from the more sterically hindered (DppNCMe)2CH ligand,
where Dpp =2,6-iPr2C6H3, by single electron oxidation of the
Cr(II) precursor, CpCr[(DppNCMe)2CH], with 1 equiv of
silver benzoate (Figure 2b). Somewhat surprisingly, CpCr-
[(DppNCMe)2CH](O2CPh), 5a, displays a Cr−O bond length
that does not differ significantly from that of 5 or the previously
reported acetate complex (see Table 1). Conversely, the Cr−N
appropriate potassium alkoxide (Scheme 3, path K). Both
alkoxide compounds 6 and 7 have similar UV−vis spectra with
absorption bands at 395, 490, and 730 nm. The solid-state
molecular structures of 6 and 7 (Figure 3) display relatively
large Cr−O−C bond angles of 151.8(2)° and 149.0(1)°,
respectively, to accommodate the large tert-butyl and CMe2Ph
groups of the alkoxide ligands. The Cr−N bond lengths for
compounds 6 and 7 (2.011(2)−2.039(2) Å) are similar to the
other Cr(III) compounds reported herein, despite the sterically
demanding alkoxide ligands, highlighting the importance of the
flexibility of the Cr−O−C bond angle in accommodating the
large alkoxide group. The Cr−O bond lengths of 6 and 7
(1.867(2) and 1.879(1) Å, respectively) are similar to the
octahedral Cr(III) alkoxide compounds [Cp′Cr(OAr)Cl]2,
where Cp′ = C5H5 or C5Me5 and Ar = 2,6-iPr2C6H3) with
Cr−O bond lengths of 1.869(2) and 1.893(1) Å.18
Manganese Reactivity with Cr(III)-X. The oxophilicity of
early first-row metals presents a challenge for catalyst design
when these strong M−O bonds must be broken to reform
reactive low-valent species. The reducing agents selected must
also be compatible with the organic substrates, their functional
groups, and the other reagents in the reaction mixture. A
particularly effective strategy developed by Furstner uses a
̈
multicomponent system with a first-row transition metal
catalyst, stoichiometric reductant, and Me3SiCl.19 This system
is useful with TiCl3 for carbonyl coupling reactions, normally
limited to stoichiometric transformations because of the strong
Ti−O bonds formed, now rendered catalytic with Me3SiCl
providing the thermodynamic driving force to cleave the Ti−O
bond, with the resulting Ti halide species being reduced with
Zn powder.20 This system was later extended to a chromium-
catalyzed Nozaki−Hiyama−Kishi (NHK) reaction using Mn as
the stoichiometric reductant of choice as it is inexpensive,
exhibits very limited reactivity with typical organic halides and
their functional groups, and the Mn halide by-products are only
weakly Lewis acidic.21,22 In their development of asymmetric
NHK catalysts, Cozzi and co-workers noted that Me3SiCl
activated the Mn, accelerating the reduction of CrCl3 to
Cr(II).23
Table 1. Selected Bond Lengths (Å) and Bond Angles (deg)
for Complexes CpCr[(XylNCMe)2CH](O2CMe), 5, and 5a
Gansauer and co-workers have also developed a similar
̈
system for epoxide ring-opening that uses catalytic Cp2TiCl2,
Mn as the stoichiometric reductant, and [HCol]Cl (2,4,6-
trimethylpyridine hydrochloride) in place of chlorosilanes to
cleave the Ti−O bond.24 This strategy was also extended to
chromium-catalyzed NHK reactions, replacing Me3SiCl with
[HCol]Cl.25 Kishi later reported the use of Cp2ZrCl2 as an
effective transmetallating reagent for the chromium-catalyzed
NHK reaction, providing increased substrate conversion
compared to Me3SiCl.26
CpCr[(XylNCMe)2CH]
(O2CMe)
5
5a
Cr−N(1)
Cr−N(2)
Cr−O(1)
O(1)−C
2.010(5)
2.004(6)
1.952(5)
1.278(9)
1.230(9)
124.9(7)
115.2(7)
119.9(7)
133.6(5)
2.0079(14)
2.0125(14)
1.9355(12)
1.286(2)
2.0233(18)
2.0359(19)
1.9309(15)
1.299(3)
O(2)−C
1.221(2)
1.227(3)
O(1)−C−O(2)
O(1)−C−C
O(2)−C−C
Cr−O(1)−C
125.69(16)
113.82(14)
120.49(15)
138.26(11)
126.0(2)
Although the Cr(III) iodide CpCr[(XylNCMe)2CH](I) was
readily reduced to the Cr(II) compound 1 by commercial Mn
powder (Scheme 3, path I),8 the corresponding bromide and
chloride complexes required the use of Mn activated with
catalytic amounts of PbBr2 or PbCl2.7 We now report that the
Cr(III) hydroxide, 4, and acetate analogue, CpCr-
[(XylNCMe)2CH](O2CMe), were also not reduced to
compound 1 without the use of PbCl2 activated Mn, while
the alkoxide compound 7 did not react even with activated Mn
at room temperature over a 24 h period.
113.3(2)
120.7(2)
137.99(15)
bonds of 5a appear to lengthen slightly, compared to the other
two compounds, as a means of alleviating steric strain in the
molecule, a feature also observed with the μ-oxo compound 3.
The Cr−O−C angle in the two benzoate compounds is ∼4°
larger than that of the acetate analogue, owing to the increased
steric requirements of the benzoate ligand.
The Cr(III) alkoxide compounds CpCr[(XylNCMe)2CH]-
(OR), where R = CMe3 (6) or CMe2Ph (7), were prepared by
salt metathesis of the Cr(III) chloride compound, 2, with the
Somewhat analogous to the system used by Gansauer,24 the
̈
μ-oxo compound 3, hydroxide 4, and alkoxide 7 were all found
to react with pyridinium halides to cleanly form the
CpCr[(XylNCMe)2CH](X) halides (Scheme 3, paths D, F,
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Figure 3. Thermal ellipsoid diagrams (50%) of (a) 6 and (b) 7. All H atoms have been omitted for clarity.
and L, respectively), which, in turn, can be reduced to the
Cr(II) compound 1 with activated Mn. Additionally, hydroxide
4 and alkoxide 7 both reacted with PhCO2H in a similar
reaction to form the benzoate compound 5.
Scheme 5
The use of Me3SiCl to activate Mn powder was successful,
providing an alternative to lead halide salts for the reduction of
the Cr(III) bromide and chloride (2) complexes. The
hydroxide compound 4 also reacted with Me3SiCl to break
the Cr−O bond, giving the chloride compound 2 (Scheme 3,
path F). Furthermore, the Cr(III) acetate compound CpCr-
[(XylNCMe)2CH](O2CMe) was readily converted to 2 by
reaction with either Me3SiCl or Cp2ZrCl2, in the latter case
generating a stoichiometric amount of Cp2Zr(Cl)(OAc)
(Scheme 3, path G).
Interestingly, the alkoxide compound 7 was found to react
with Cp2ZrCl2, as expected, to form compound 2 (Scheme 3,
path L), but 7 was found to be completely unreactive toward
Me3SiCl (2 days at room temperature) under anhydrous
conditions. However, an identical reaction conducted with an
additional 0.5 equiv of H2O led to conversion of alkoxide 7 to
chloride 2 (UV−vis). It has been proposed by Wessjohann that
Cr(III) alkoxides do not react directly with Me3SiCl.27
Additionally, Jacobsen reported that in the epoxide ring-
opening reactions with Me3SiN3 catalyzed by Cr(III), the strict
exclusion of water resulted in deactivation of the catalytic
system while reactivation could be achieved with the addition of
trace water, causing the formation of small amounts of HN3.28
It was therefore proposed that HN3, and not Me3SiN3, was, in
fact, the active species for the key Cr(III)−OR bond cleavage
step. This mechanism is consistent with the observed reactivity
of the Cr(III) alkoxide compound 7 with Me3SiCl in the
presence of added water.
Treatment of the hydroxide compound 4, the Cr(III) acetate
compound CpCr[(XylNCMe)2CH](O2CMe), and the Cr(III)
enolate compound CpCr[(XylNCMe)2CH][OC(=CH2)Ph]
with in situ generated MnI2 resulted in transmetalation to
form the Cr(III) iodide complex (Scheme 3, paths F, G, and M,
respectively),29 which if conducted in the presence of excess
Mn powder results in direct reduction to the Cr(II) compound
1.30 The μ-oxo compound 3 was also found to react with
MnX2, where X = Cl or I, to form the appropriate Cr(III)
halide species. Alternatively, the alkoxide compound 7 did not
react with MnI2 to form the Cr(III) iodide, potentially a
contributing factor to the inability of activated Mn to reduce
compound 7.
[(XylNCMe)2CH](I) and the 31P NMR spectrum contained
only one peak corresponding to OPPh3.
The oxidation of PPh3 was investigated to assess the stability
of the cyclopentadienyl chromium β-diketiminate framework
under catalytic oxidative conditions and to compare the
reactivity of μ-oxo 3 with other oxygen atom transfer (OAT)
catalysts. Many of the examples of chromium catalyzed OAT
involve a Cr(III)/Cr(V) redox couple where the Cr(III) must
be sufficiently reducing to form a Cr(V) oxo, which then must
be sufficiently oxidizing to react with organic substrates to
achieve the oxo transfer.31 The Cr corrole system of Gray and
co-workers is an example of the Cr(III)/Cr(V) oxo redox
couple; the Cr(V) oxo reacts quickly with PPh3 to generate
OPPh3 but the Cr(III) is slow to react with O2.12h This system
also suffers from product inhibition where the reduced Cr(III)
corrole coordinates OPPh3, further reducing the rate of Cr(III)
oxidation. Veige and co-workers adopted a tridentate pincer
ligand that was found to increase the rate of Cr(III) oxidation
by O2, as a result of the open coordination site on the Cr
center, created by moving from a tetra- to a tridentate ligand
framework.12k
Alternatively, using a Cr(II)/Cr(IV) oxo redox couple
presents two potential advantages: the increased reactivity of
the reduced Cr(II) toward oxidation with O2 and the expected
higher reactivity of the Cr(IV) oxo species, compared to a
Cr(V) analogue. The latter observation was reported for the Cr
corrole system where single-electron reduction of the Cr(V)
oxo compound of Gray and co-workers to an anionic Cr(IV)
oxo species was proposed to result in both an increased
electrophilicity and an increase in the unpaired electron density
of the oxygen atom.12j Similarly, in situ generated aqueous
OAT Reactivity: PPh3 Oxidation. The μ-oxo compound 3
rapidly reacted with Ph3PI2 causing a color change from orange
to green (Scheme 5). The UV−vis spectrum of the reaction
mixture was consistent with the formation of CpCr-
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CrO2+ reacted with PPh3 to form OPPh3 with a rate constant of
2.1(2) × 103 M−1 s−1.32
It was thought that a catalytic system could be achieved from
a Cr(II) CpCr[(ArNCMe)2CH] compound. As discussed
above, the Cr(II) compound CpCr[(XylNCMe)2CH], 1,
reacted rapidly with O2 to form the bimetallic Cr(III) μ-oxo
compound 3. Additionally, 1 does not readily coordinate
OPPh3 or other Lewis bases, avoiding the issue of product
inhibition.11f,12h Unfortunately, the μ-oxo compound 3 did not
react with PPh3, presumably as a result of the highly stable
nature of the μ-oxo core. The formation of 3 is thought to
proceed through a terminal Cr(IV) oxo species: the monomeric
CpCr[(XylNCMe)2CH](O) oxo intermediate would be
expected to be much more reactive toward OAT reactions.
By replacing the 2,6-dimethylphenyl groups of the β-
diketiminate ligand with slightly larger 2,6-diethylphenyl the
stability of the Cr(III) μ-oxo species should be reduced due to
the increased bulk being directed at the already strained Cr−
O−Cr core, leading to an increased likelihood of a Cr(IV) oxo.
The Cr(II) compound CpCr[(DepNCMe)2CH], 1a, was, in
fact, found to catalyze the aerobic oxidation of PPh3 to OPPh3
(eq 2). Exposure of a solution of 1a with a large excess of PPh3
Figure 4. Thermal ellipsoid diagram (50%) of 4. H atoms, with the
exception of the hydroxide group, have been omitted for clarity.
Disordered Cp and hydroxide ligands were each modeled in two
orientations; in both cases, only one is shown.
Cr(II) complex 1 with pyridine N-oxide may be reversible. The
steric demands of the 2,6-dimethylphenyl substituted β-
diketiminate ligand apparently aid in the reversible dissociation
of μ-oxo 3, analogous to the Cr porphyrin system described by
Liston and West.15 However, the proposed Cr(IV) oxo species
generated is rapidly trapped by Cr(II) complex 1 in the absence
of a substrate with weak C−H bonds. Similarly, the Cr(II)
complex 1 does not react with γ-terpinene; so in the absence of
oxidant, only a minute amount of Cr(III) hydroxide product 4
is presumably formed before the build up of complex 1
effectively precludes further consumption of μ-oxo 3.
Significant conversion of μ-oxo 3 to the HAT product Cr(II)
hydroxide 4 is only achieved when both γ-terpinene and
pyridine N-oxide are present. No further reaction of compound
4 was observed with the excess γ-terpinene.
The HAT reactivity ascribed to the putative chromium oxo
species in Scheme 6 is consistent with the expected strength of
the O−H bond in 4, the Cr(III) hydroxide product.33 This
rationale has been used to explain the radical reactivity of
aqueous CrO2+,34 as well as biologically relevant Cr(IV) and
Cr(V) complexes.35 Work with iron imido complexes have
examined the role of N-based radical character on HAT
reactivity.36 We were therefore interested in isolating an
analogue of the monomeric oxo intermediate in Scheme 6 to
evaluate the degree of oxo-based radical character37 and its
possible impact on reactivity.38
Reaction of the bulkier 2,6-diethylphenyl Cr(II) compound
1a with pyridine N-oxide or O2 resulted in a rapid color change
to orange, consistent with Cr(III) μ-oxo formation, followed by
a further color change to green. The decrease in stability of the
Cr(III) μ-oxo with increased steric bulk of the β-diketiminate
ligand is consistent with the observed PPh3 oxidation reactivity,
suggesting a Cr(IV) oxo intermediate is the active OAT species
in solution. The UV−vis spectrum of the green crude reaction
mixture was consistent with the formation of a Cr(III)
hydroxide compound, but no tractable products could be
isolated from the reactions in a variety of different solvents.
Addition of the hydrogen atom source γ-terpinene to the
reaction of the Cr(II) compound 1a with pyridine N-oxide,
analogous to the reaction of 1 to form hydroxide 4, cleanly
provided the Cr(III) hydroxide compound CpCr-
[(DepNCMe)2CH](OH), 4a (Scheme 7). Compound 4a was
more readily prepared compared to 4, giving a higher yield with
reduced reaction time, also consistent with the decreased
stability of the bimetallic Cr(III) μ-oxo species when the larger
β-diketiminate ligand framework is present.
to one atmosphere of O2 resulted in the formation of OPPh3
with a maximum turnover number (TON; mol product/mol
catalyst) of 20, based on isolated OPPh3. Air was also found to
be an effective source of oxygen for this reaction, as 31P NMR
analysis showed complete conversion of 6 equiv of PPh3 upon
air exposure over a period of approximately 10 min. TONs
were ultimately limited under the reaction conditions as a result
of catalyst deactivation.
HAT Reactivity. As discussed above, the μ-oxo complex 3
did not react with excess pyridine N-oxide in THF.
Alternatively, 3 was found to react with an excess of pyridine
N-oxide in the presence of the hydrogen atom source γ-
terpinene over a 2 day period at room temperature. Notably,
complex 3 did not react with γ-terpinene in the absence of
pyridine N-oxide, highlighting the need for an oxidant in the
reaction. UV−vis analysis of the crude reaction mixture
suggested that the product of the reaction was the Cr(III)
hydroxide compound 4. The solid-state molecular structure
obtained from recrystallized 4 confirmed the intermolecular
HAT, showing that the product of the reaction was the
hydroxide complex 4 (Figure 4). The Cr−O bond length of
1.896(1) Å is slightly longer than the 1.814(2) Å reported for
the cationic 5-coordinate Cr(III) hydroxide complex
[TptBu,MeCr(OH)(pz′H)], prepared from the Cr(IV) oxo
precursor undergoing a hydrogen atom abstraction from
substrates containing C−H bond dissociation energies
(BDEs) < 92 kcal/mol.12g Related hydrogen atom abstraction
reactions of cyclohexadiene with well-defined Cr(V) oxo and
Cr(III) superoxo complexes have been reported by Nam and
co-workers.13b,c
A possible mechanism for the observed HAT reactivity is
shown in Scheme 6. The lack of further oxidation of μ-oxo 3
with excess pyridine N-oxide suggests that the reaction of
The formation of Cr(III) hydroxide species in the absence of
γ-terpinene suggested that intermolecular HAT from solvent or
even intramolecular HAT from the ligands was the route to the
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Inorganic Chemistry
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Scheme 6
bond lengths of 1.8569(13) Å and 1.8654(13) Å and Cr−N
bond lengths in the range of 2.0132(14)−2.0282(13) Å.
In the absence of external hydrogen atom sources, the
Cr(IV) oxo intermediate underwent intramolecular HAT from
the secondary benzylic groups of the β-diketiminate ligand
followed by intermolecular radical C−C bond formation to
generate the Cr(III) hydroxide dimer 8. Examples of ligand
based C−H activation of diiminepyridine backbone methyl
groups resulting in intermolecular C−C coupling have been
previously reported.39 An example of intramolecular HAT from
a β-diketiminate ligand was reported by Holland and co-
workers with an imido Fe(III) compound that results in
intramolecular C−C bond formation.36b
Scheme 7
Synthesis of {CpCr[(HIPTNCMe)2CH]}2(μ-O). Replacing
the ortho substituents on the N-aryl groups of the β-
diketiminate ligand with meta substituents removes the reactive
benzylic C−H bonds in proximity to the Cr−O reactive site.
The size of bulky substituents must be substantially increased
when placed in meta positions if bimetallic μ-oxo formation is
to be discouraged. Piers used the hexaisopropylterphenyl
{HIPT, [3,5-(2,4,6-iPr3C6H2)2C6H3]} substituents pioneered
by Schrock40 to prevent unwanted C−H activation of the β-
diketiminate ligand.41
Cr(III) hydroxide formation. Compound 1a was reacted with
pyridine N-oxide in benzene to remove the possibility of
solvent based HAT (Scheme 7). The isolated product was
characterized by X-ray crystallography as a Cr(III) hydroxide
dimer, compound 8 (Figure 5). Compound 8 has similar bond
lengths to the monometallic Cr(III) hydroxide 4 with Cr−O
Preparation of the Cr(II) compound CpCr-
[(HIPTNCMe)2CH] was achieved by the standard method
of reacting CrCl2 with 1 equiv of NaCp followed by 1 equiv of
the deprotonated β-diketiminate ligand to form a solution that
was green to incident and magenta to transmitted light, similar
to that of the Cr(II) compound 1. The high solubility of the
complex in hexanes precluded isolation by recrystallization. The
CpCr[(HIPTNCMe)2CH] compound reacted with pyridine
N-oxide to form an orange solution with an absorption in the
UV−vis spectrum at 371 nm and a shoulder at 475 nm,
analogous to the μ-oxo compound 3 (Scheme 8). The solid-
state molecular structure obtained from the isolated orange
crystalline material exhibited extensive disorder and low data
quality, as has previously been observed in metal complexes
bearing ligands with HIPT substituents.40c,d The complex also
crystallized with unresolvable residual electron density, likely
from disordered solvent in the lattice. Due to these difficulties,
the full composition of compound 9 is unknown, and the
specific bond lengths and angles for 9 may be somewhat
unreliable. However, the structure confirmed that despite the
Figure 5. Thermal ellipsoid diagram (50%) of 8. H atoms, with the
exception of the hydroxide groups, have been omitted for clarity.
Compound 8 crystallizes with two independent molecules in the
asymmetric unit, each residing on a 2-fold axis of rotation.
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Inorganic Chemistry
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Scheme 8
introduction of large meta substituents, 9 has a bimetallic
Cr(III) μ-oxo core (Figure 6).
Haloacetal Cyclization. We reported a chromium-cata-
lyzed radical cyclization of bromo and chloro acetals.
Cyclization of the bromo substrates was achieved in high
yields with only 2 mol % Cr catalyst loadings (Table 2, entry
Table 2. Chromium-Catalyzed Bromoacetal Radical
a
Cyclization
b
entry
Cr Catalyst
additives
yield
1
2
3
4
2 mol % Cr(III)-Br <1 mol % PbBr2
93% (82/18)
<40%
1 mol % 3
1 mol % 3
1 mol % 3
1 mol % PbBr2
c
3 mol % [HLut]Br
83% (82/18)
90% (82/18)
3 mol % [HLut]Br,
1 mol % PbBr2
d
e
5
6
2 mol % Cr(III)-Br 10 mol % Me3SiCl
2 mol % Cr(III)-Br 10 mol % Me3SiCl
87% (82/18)
92% (80/20)
Figure 6. Thermal ellipsoid diagram (50%) of 9. All H atoms and iso-
propyl groups have been omitted for clarity.
a
Reaction conditions: substrate (1 mmol), Mn (2 equiv), γ-terpinene
b
(4 equiv), THF (4 mL), 38.5 h at 50 °C. Isolated yields.
Diastereomeric ratios are in parentheses. Compound 3 mixed with
3 mol % [HLut]Br and Mn (2 equiv) for 4 h prior to addition of
substrate. No reaction is observed without premixing step. 24 h
reaction time. Reagents were mixed under air prior to reaction for 22
h at 65 °C.
c
Compound 9 exhibits a Cr−O−Cr angle (174.3(2)°) that is
essentially identical to the μ-oxo compound 3 (174.5(2)°). The
Cr−O bond lengths (1.784(4) and 1.786(3) Å), in contrast, are
significantly shorter compared to 3 (1.834(3) Å). Additionally,
reactivity studies of {CpCr[(HIPTNCMe)2CH]}2(μ-O), 9,
suggested that the compound was actually less reactive than
compound 3. Unlike 3, compound 9 did not react with H2O
nor was there any reaction with excess pyridine N-oxide and γ-
terpinene upon stirring for 10 days. The lack of protonolysis
and HAT reactivity in combination with the shorter Cr−O
bond lengths indicated that there was no significant increase in
steric strain of the bimetallic μ-oxo species between the HIPT
and Xylyl derivatives of the β-diketiminate ligand.
d
e
1).7 The active catalyst species in the reaction, compound 1, is
extremely air-sensitive, forming the Cr(III) μ-oxo compound 3
in the presence of oxygen. As a result, the halo acetal reactions
must be performed with the strict exclusion of oxygen. We now
report the use of μ-oxo compound 3 as a catalyst precursor and
present conditions that allow for compound 3 to be converted
back to the active catalyst under catalytically relevant reaction
conditions.
Catalytic HAT. Attempts to render a catalytic HAT
reaction based on a Cr(IV) oxo catalyst with O2 as the
oxidant, founded on the stoichiometric reactivity discussed
above, proved challenging. When Mn was used as the
stoichiometric reductant the catalyst turnovers were ultimately
limited by one of two factors: either the Mn was deactivated by
the presence of the oxidant and would no longer reduce the
Cr(III) intermediates, as was the case with PbCl2 activated Mn,
or the oxidant was rapidly consumed by the activated Mn to
form Mn oxides, as was the case with I2 or Me3SiCl activating
agents.42 The best result for the catalytic in Cr conversion of γ-
terpinene to p-cymene was limited to 4 turnovers of the catalyst
(2 equiv of p-cymene per Cr). The optimal reaction conditions
were found with a catalytic amount of CpCr-
[(DepNCMe)2CH](Cl), 2a, excess Zn as the stoichimetric
reductant, and either Me3SiCl or PbCl2 as the activating agent
for the Zn. A discussion of the attempted HAT reactivity is
presented in the Supporting Information.
In a control experiment, the direct use of compound 3 as
catalyst precursor under the standard reaction conditions led to
<40% conversion (entry 2), and no substrate conversion was
observed with 1 mol % of 3 if the PbBr2 additive was replaced
with 3 mol % of lutidinium bromide. However, the μ-oxo
compound 3 was readily reduced to 1 if 2 equiv of [HLut]Br
and an excess of Mn were added before the haloacetal and γ-
terpinene (Scheme 3, paths D and I). Subsequent addition of
the desired bromo substrate under standard reaction conditions
resulted in an 83% isolated yield (Table 2, entry 3), only a 10%
decrease in yield compared to the standard conditions using the
CrCr[(XylNCMe)2CH](Br) catalyst. Entry 4 shows the
optimized reaction conditions with only 1 mol % of the μ-
oxo 3 providing 90% isolated yield of the desired cyclized
product.
It was also determined that a substoichiometric amount of
Me3SiCl could be used to activate the Mn in place of the PbBr2
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Table 3. X-ray Crystallographic Data for Compounds 3, 4, 5, and 5a
3·1/2C6H14
4
5
5a
formula
C55H67N2Cr2O
904.13
C26H31N2OCr
439.53
C33H35N2O2Cr
543.63
C41H51N2O2Cr
655.84
formula weight
crystal color, habit
crystal dimensions, mm
crystal system
space group
a, Å
black, needle
0.03 × 0.15 × 0.15
triclinic
black, prism
0.20 × 0.30 × 0.35
monoclinic
P 21/n
black-green, prism
0.25 × 0.30 × 0.50
triclinic
dark green, plate
0.25 × 0.35 × 0.50
monoclinic
P 21/c
P1
̅
P1
̅
9.1063(8)
14.7309(14)
18.546(2)
90.341(3)
95.604(4)
107.903(4)
2354.4(4)
2
12.0157(3)
15.0102(4)
13.2704(3)
90
9.5648(10)
12.8067(15)
13.3869(16)
63.185(6)
73.615(6)
82.412(6)
1404.0(3)
2
17.658(2)
10.735(2)
19.833(4)
90
b, Å
c, Å
α, deg
β, deg
109.716(1)
90
91.067(7)
90
γ, deg
V, Å3
2253.1(1)
4
3768(1)
4
Z
Dcalc, g/cm3
1.275
1.296
1.286
1.156
F000
960
932
574
1404
μ(Mo Kα), cm−1
data images (no., t/s)
2θmax
5.05
5.27
4.40
3.38
832, 25
45.0°
1067, 10
60.1°
2344, 5
56.2°
3106, 5
56.0°
reflcns measrd
unique reflcn, Rint
absorption, Tmin, Tmax
obsrvd data (I > 2.00σ(I))
no. parameters
R1, wR2 (F2, all data)
R1, wR2 (F, I > 2.00σ(I))
goodness of fit
max, min peak, e−/Å3
16713
25873
29288
172767
5987, 0.083
0.643, 0.985
3478
6591, 0.027
0.821, 0.900
5605
6734, 0.029
0.704, 0.896
5627
16687, 0.082
0.671, 0.919
8110
572
357
349
426
0.127, 0.113
0.052, 0.089
0.99
0.043, 0.100
0.034, 0.094
1.04
0.048, 0.102
0.038, 0.096
1.04
0.096, 0.135
0.049, 0.124
1.01
0.34, −0.42
0.47, −0.33
0.35, −0.41
0.40, −0.58
under the standard reaction conditions, with only a slight
decrease in yield (entry 5). Most notable is the result in entry 6,
where the reaction vessel was charged with the reagents and
solvent under ambient atmosphere, after which the headspace
of the reaction vessel was purged with N2 before being sealed.
The reaction was heated to 65 °C for 22 h to afford a 92%
isolated yield upon workup.
The success of the reaction can be attributed to the fact that
Me3SiCl activated Mn reacts rapidly with O2 to form Mn
oxides, the same reason the Mn was not effective for the HAT
reactions. The activated Mn scrubbed the reaction of trace
oxygen before the Cr(III) bromide catalyst was reduced to the
air-sensitive Cr(II) state, thereby alleviating the need to
meticulously exclude trace oxygen from the haloacetal
cyclization reactions.
conditions is expected to make Cr-based catalysts more widely
applicable.22,43
Despite their complexity, multicomponent catalytic systems
are remarkably amenable to mechanistic interrogation, as each
individual step can be investigated separately. The study of the
paramagnetic Cr(III) compounds described in this paper was
assisted by the crystallinity imparted by the cyclopentadienyl
and β-diketiminate ancillary ligands, and by the distinctive color
changes that accompany the variation of the X group in the
Cr(III) CpCr[(ArNCMe)2CH](X) complexes. The μ-oxo
dimer was readily converted to Cr(III)−X species using
reagents with known compatibility with Mn as a stoichiometric
reductant. Replacing PbBr2 with 10 mol % Me3SiCl allowed the
previously reported Cr-catalyzed radical cyclization of a
bromoacetal to be set up under ambient conditions. The
CpCr[(ArNCMe)2CH] framework proved to be remarkably
robust with respect to air oxidation, protonolysis, salt-
metathesis, transmetalation, and reduction reactions.
In the presence of pyridine N-oxide, the μ-oxo dimer
underwent an intermolecular HAT reaction with γ-terpinene.
This reactivity was ascribed to a putative monomeric oxo
intermediate and is consistent with both the expected strength
of the O−H bond of the Cr(III) hydroxide product and with
possible oxo-based radical character. The use of very large
HIPT substituents on the β-diketiminate ancillary ligand did
not prevent μ-oxo dimer formation. However, changing the N-
aryl ortho substituents from methyl to ethyl did permit the Cr-
catalyzed aerobic oxidation of PPh3. In the absence of OAT or
HAT substrates, a product consistent with intramolecular
ligand HAT followed by radical C−C bond formation was
structurally characterized.
CONCLUSIONS
■
Although potent single-electron reductants such as Cr(II),
Sm(II),9 and Ti(III)10 are used for the stoichiometric
generation of carbon-based radicals, several challenges must
be overcome before these highly air-sensitive reagents can be
employed catalytically. The strong metal−oxygen bonds in the
products must be broken, and a sufficiently powerful
stoichiometric reductant is required to reform the active low-
valence species. Furstner’s Me SiCl/Mn multicomponent
̈
3
system made ancillary ligand development feasible for
asymmetric NHK reactions catalytic in chromium.21 While
high-spin monomeric Cr(II) compounds are intrinsically air-
sensitive, understanding how reactive species can be regen-
erated from air-oxidized complexes under catalytically relevant
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Table 4. X-ray Crystallographic Data for Compounds 6, 7, 8, and 9
6
7
8
9
formula
C30H39N2OCr
495.63
C35H41N2OCr
439.53
C60H76N4O2Cr2
989.25
C164H220N4OCr2
2367.44
formula weight
crystal color, habit
crystal dimensions, mm
crystal system
space group
a, Å
black, prism
0.135 × 0.15 × 0.16
monoclinic
P21/n
Black-green, plate
0.03 × 0.50 × 0.50
monoclinic
P21/n
black, prism
0.10 × 0.17 × 0.25
monoclinic
P2/c
orange, prism
0.125 × 0.16 × 0.22
monoclinic
P21/c
16.302(3)
9.0161(16)
18.449(3)
90
11.5614(10)
8.9497(8)
29.309(3)
90
16.0439(8)
11.9799(5)
27.4399(13)
90
31.054(3)
18.839(2)
31.163(3)
90
b, Å
c, Å
α, deg
β, deg
99.280(4)
90
99.964(5)
90
103.200(2)
90
111.582(2)
90
γ, deg
V, Å3
2676.2(8)
4
2986.9(5)
4
5134.7(6)
4
16953(3)
4
Z
Dcalc, g/cm3
1.230
1.240
1.280
0.928
F000
1060
1188
2112
5152
μ(Mo Kα), cm−1
data images (no., t/s)
2θmax
4.52
4.12
4.71
1.72
967, 60
1173, 10
56.0°
1177, 10
60.1°
824, 90
48.3°
45.1°
reflcns measrd
unique reflcn, Rint
absorption, Tmin, Tmax
obsrvd data (I > 2.00σ(I))
no. parameters
R1, wR2 (F2, all data)
R1, wR2 (F, I > 2.00σ(I))
goodness of fit
max, min peak, e−/Å3
15747
32185
58708
83553
4261, 0.041
0.866, 0.941
3313
7184, 0.037
0.856, 0.988
5348
15030, 0.046
0.895, 0.954
10647
21958, 0.127
0.738, 0.979
12172
312
360
617
1592
0.066, 0.112
0.044, 0.102
1.06
0.067, 0.112
0.042, 0.101
1.03
0.076, 0.122
0.046, 0.108
1.02
0.145, 0.231
0.085, 0.204
0.949
0.34, −0.35
0.31, −0.36
0.50, −0.35
0.63, −0.56
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
using the Bruker SAINT software package48 and were corrected for
absorption effects using the multiscan technique (SADABS).49 The
data were corrected for Lorentz and polarization effects. All structures
were solved by direct methods.50 All non-hydrogen atoms were refined
anisotropically (unless otherwise mentioned below). All hydrogen
atoms were placed in calculated positions but not refined. All
refinements were performed using the SHELXTL crystallographic
software package of Bruker-AXS.51 The molecular drawings were
generated by the use of ORTEP-352 and POV-Ray. X-ray crystallo-
graphic data for compounds 3, 4, 5, and 5a are shown in Table 3. X-ray
crystallographic data for compounds 6, 7, 8, and 9 are shown in Table
4.
Compound 3 crystallizes with one-half-molecule of hexanes
(residing on an inversion center) in the asymmetric unit. Both the
cyclopentadienyl ligand and the hydroxide ligand of compound 4 were
disordered: each was subsequently modeled in two orientations.
Compound 8 crystallizes with two independent molecules in the
asymmetric unit, each residing on a 2-fold axis of rotation.
Compound 5 crystallizes as a two-component twin, with the second
component related to the first by a rotation of 180° about the axis
perpendicular to the (001) plane. The data was integrated for both
components, including both overlapping and non-overlapping data.
Subsequent refinements were carried out using the HKLF 5 format
data set containing all reflections from both twin components. The
fraction of the second twin component refined to a value of 0.406(1).
Compound 9 crystallizes with unresolvable residual electron
density, likely from disordered solvent, in the lattice. The structure
was refined without modeling any solvent molecules, then the
PLATON/SQUEEZE53 program was employed to search the cell
for solvent accessible voids, and then to correct the X-ray diffraction
data to eliminate any residual electron density found in those voids.
The result from this procedure removed 840 residual electron density
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise indicated, all reac-
tions were carried out under nitrogen using standard Schlenk and
glovebox techniques. Hexanes, diethylether, CH2Cl2, and THF were
purified by passage through activated alumina and deoxygenizer
columns from Glass Contour Co. (Laguna Beach, CA). Celite
(Aldrich) was dried overnight at 120 °C before being evacuated and
then stored under nitrogen. γ-Terpinene was degassed by three
freeze−vacuum−thaw cycles and stored under nitrogen prior to use.
C6D6 was dried over sodium/benzophenone, purified by vacuum
distillation, degassed by three freeze−vacuum−thaw cycles, and stored
under nitrogen. CDCl3 was used as received.
Pyridine N-oxide, benzoic acid, AgO2CPh, and KOtBu were used as
received. KOC(Me)2Ph was prepared by reacting HOC(Me)2Ph with
1 equiv of KN(SiMe3)2 in 10 mL of hexanes/toluene (5:1), while
stirring for 2 h. The product was isolated by vacuum filtration, rinsed
with hexanes, dried under vacuum, and stored under N2. CpCr-
[(XylNCMe)2CH], (1),44 CpCr[(DepNCMe)2CH], (1a),17 CpCr-
[(DppNCMe)2CH], (1b),45 CpCr[(XylNCMe)2CH](Cl), (2),17 and
46
Ph3PI2 were prepared according to the literature procedures.
H[(HIPTNCMe)2CH] was prepared with a slight modification to
the literature procedure.41
1H and 31P spectra were recorded on a Varian Mercury Plus 400
spectrometer. UV−visible spectroscopic data was collected on Varian
Cary 100 Bio or Shimadzu UV-2550 UV−visible spectrophotometers
in a specially constructed cell for air-sensitive samples: a Kontes Hi-
Vac Valve with PTFE plug was attached by a professional glassblower
to a Hellma 10 mm path length quartz absorption cell with a quartz-to-
glass graded seal. Elemental analyses were performed by Guelph
Chemical Laboratories, Guelph, ON, Canada or by the UBC
Department of Chemistry microanalytical services. Solution magnetic
susceptibilities were determined by the Evans method.47
X-ray Crystallography. All crystals were mounted on a glass
fiber, and measurements were made on a Bruker X8 APEX II
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Inorganic Chemistry
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from the unit cell, or approximately 210 electrons per asymmetric unit.
Because it is not possible to properly identify the solvent, the values for
the formula weight, etc., reflect only those atoms found in the atom
list.
mmol, 1.17 equiv) suspended in Et2O (3 mL). After stirring for 4 days,
the volatiles were removed in vacuo; the residue was extracted with
hexanes and filtered over Celite. The solution was concentrated to a
volume of approximately 0.5 mL and cooled to −35 °C to provide
black crystals of 6 (24.0 mg, 42%) in two crops. Anal. calcd. for
C30H39N2CrO: C, 72.70; H, 7.93; N, 5.65. Found: C, 71.16; H, 7.93;
N, 5.58. UV−vis (hexanes; λmax, nm (ε, M−1 cm−1)): 393 (8620), 492
(880), 725 (550).
Preparation of {CpCr[(XylNCMe)2CH]}2(μ-O) (3). CpCr-
[(XylNCMe)2CH] (1) (495 mg, 1.17 mmol) was placed in a
round-bottom flask followed by the addition of 61.8 mg (0.65 mmol,
0.55 equiv) of pyridine N-oxide and approximately 10 mL of hexanes.
After stirring overnight, the orange precipitate was isolated, rinsed with
a small amount of diethyl ether, and dried to produce an orange
powder (408 mg, 81%). μeff (Evans, CDCl3) = 2.39 μβ. Anal. calcd. for
C52H60N2Cr2O: C, 72.53; H, 7.02; N, 6.51. Found: C, 72.87; H, 6.87;
N, 6.96. UV−vis (hexanes; λmax, nm (ε, M−1 cm−1)): 362 (20700), 471
(sh).
Preparation of CpCr[(XylNCMe)2CH](OH) (4). Compound 1
(100 mg, 0.237 mmol) was placed in a Schlenk flask followed by the
addition of THF, 200 μL (1.25 mmol, 5.25 equiv) of γ-terpinene, and
110 mg (1.16 mmol, 4.89 equiv) of pyridine N-oxide. The solution
quickly turned orange, and upon stirring for 2 days, it turned green, at
which point the volatiles were removed in vacuo. The residue was
extracted with hexanes, filtered over Celite, and the solvent was again
removed in vacuo. The residue was extracted with a minimum amount
of hexanes (3 mL), filtered, and cooled to −35 °C to provide black
crystals of 4 (36 mg, 35%) in three crops. Anal. calcd. for
C26H31N2CrO: C, 71.05; H, 7.11; N, 6.37. Found: C, 70.94; H,
7.11; N, 6.50. UV−vis (hexanes; λmax, nm (ε, M−1 cm−1)): 387 (9280),
503 (730), 614 (730).
Preparation of CpCr[(XylNCMe)2CH][OC(Me)2Ph] (7). Com-
pound 2 (59.8 mg, 0.131 mmol) was placed in a Schlenk flask followed
by the addition of THF (7 mL). The addition of a 2 mL THF solution
of KOC(Me)2Ph (23.2 mg, 0.133 mmol, 1.02 equiv) caused a rapid
color change from green to green−orange. After stirring for 5 days, the
volatiles were removed in vacuo; the residue was extracted with
hexanes and filtered over Celite. The solution was concentrated to a
volume of approximately 0.5 mL and cooled to −35 °C to provide
black crystals of 7 (50.7 mg, 70%). Anal. calcd. for C35H41N2CrO: C,
75.38; H, 7.41; N, 5.02. Found: C, 73.81; H, 7.46; N, 5.04. UV−vis
(hexanes; λmax, nm (ε, M−1 cm−1)): 395 (8920), 491 (640), 740 (450).
Preparation of {CpCr[(DepNCMe)2CH](OH)}2 (8). CpCr-
[(DepNCMe)2CH] (1a) (50.2 mg, 0.105 mmol) and pyridine N-
oxide (10.7 mg, 0.113 mmol, 1.1 equiv) were placed in a round-
bottom flask, followed by the addition of benzene (5 mL). The
reaction mixture was stirred at room temperature overnight, followed
by removal of the solvent in vacuo. The residue was extracted with
hexanes (1 mL), filtered over Celite, and cooled to −35 °C to provide
a black microcrystalline solid (17.7 mg) in two crops. A third crop of
crystals (4.4 mg, 8%) suitable for X-ray crystallography was isolated
and identified as compound 8. The UV−visible spectrum of 8 was
essentially indistinguishable from the corresponding monomeric
Cr(III) hydroxide complex 4a: UV−vis (hexanes; λmax, nm): 388,
503, 614.
Preparation of {CpCr[(HIPTNCMe)2CH]}2(μ-O) (9). H-
[(HIPTNCMe)2CH] (136 mg, 0.129 mmol, 1.06 equiv) was placed
in a Schlenk flask, dissolved in THF (5 mL), and cooled to −35 °C. n-
BuLi (0.09 mL of 1.6 M solution in hexanes, 0.14 mmol, 1.1 equiv)
was added dropwise, and the resulting yellow solution was allowed to
warm to room temperature while stirring for 0.5 h. In a separate
Schlenk flask, CrCl2 (15.0 mg, 0.122 mmol) was suspended in THF (5
mL), followed by the addition of NaCp (0.7 mL of 2.0 M solution in
THF, 0.14 mmol, 1.1 equiv); after stirring at room temperature for 0.5
h, the Li[(HIPTNCMe)2CH] prepared above was added. The
resulting green to incident and magenta to transmitted light solution
was stirred overnight, followed by the removal of the solvent in vacuo.
The residue was extracted with hexanes and filtered over Celite
followed by the addition of pyridine N-oxide (12.2 mg, 0.128 mmol,
1.05 equiv), causing the reaction mixture to turn orange over a 5 min
period. After stirring overnight at room temperature, the solvent was
removed in vacuo; the residue was extracted with hexanes, filtered over
Celite, concentrated to a volume of approximately 0.5 mL, and cooled
to −35 °C to provide a small amount of orange crystalline material
(4.5 mg), identified as compound 9 by X-ray crystallography. UV−vis
(hexanes; λmax, nm): 371, 475 (sh).
Preparation of CpCr[(DepNCMe)2CH](OH) (4a). Compound
4a was prepared in a similar manner to compound 4 with
CpCr[(DepNCMe)2CH] (1a) (94.1 mg, 0.197 mmol), γ-terpinene
(300 μL, 1.86 mmol, 9.44 equiv), and pyridine N-oxide (36.9 mg,
0.388 mmol, 1.97 equiv). The reaction mixture was stirred overnight.
Workup and cooling to −35 °C in hexanes (2 mL) provided 57.3 mg
(59%) of 4a. Anal. calcd. for C30H39N2CrO: C, 72.70; H, 7.93; N, 5.65.
Found: C, 71.18; H, 7.99; N, 5.64. UV−vis (hexanes; λmax, nm (ε,
M
−1 cm−1)): 388 (10400), 503 (740), 614 (760).
Preparation of CpCr[(XylNCMe)2CH](O2CPh) (5). Method
A. Compound 2 (49.3 mg, 0.108 mmol) and AgO2CPh (24.4 mg,
0.107 mmol, 1.0 equiv) were placed in a Schlenk flask, followed by the
addition of CH2Cl2 (20 mL). After stirring overnight, the volatiles
were removed in vacuo; the residue was extracted with Et2O (25 mL)
and filtered under ambient atmosphere. The volatiles were again
removed in vacuo; the residue was extracted with approximately 15 mL
of hexanes/Et2O (3:2) and concentrated, and the resulting green
solution was cooled to −20 °C to provide black crystals of 5 (34.5 mg,
60%) in two crops. Anal. calcd. for C33H35N2O2Cr: C, 72.91; H, 6.49;
N, 5.15. Found: C, 73.02; H, 6.33; N, 5.18. UV−vis (hexanes; λmax, nm
(ε, M−1 cm−1)): 413 (9320), 503 (680), 584 (770).
Method B. Compound 3 (119.5 mg, 0.139 mmol) and benzoic acid
(34.4 mg, 0.282 mmol, 2.03 equiv) were placed in a Schlenk flask,
followed by the addition of THF (30 mL). The solution changed color
from orange to green and was stirred for 3 days; at which point, the
volatiles were removed in vacuo, and the residue was extracted with
approximately 5 mL of hexanes and 2 mL of CH2Cl2, concentrated,
and cooled to −20 °C to provide 5 (98.7 mg, 66%) in two crops with
an identical UV−visible spectrum to the product obtained from
method A.
Preparation of CpCr[(DppNCMe)2CH](O2CPh) (5b). Com-
pound 1b (198 mg, 0.370 mmol) and AgO2CPh (85.7 mg, 0.374
mmol, 1.01 equiv) were placed in a Schlenk flask followed by the
addition of THF (15 mL). After stirring overnight, the volatiles were
removed in vacuo; the residue was extracted with hexanes and filtered
over Celite. The green solution was concentrated to a volume of
approximately 2 mL and cooled to −35 °C to provide black crystals of
5b (106 mg, 43%). Anal. calcd. for C35H41N2CrO: C, 75.08; H, 7.84;
N, 4.27. Found: C, 74.98; H, 7.95; N, 4.21. UV−vis (hexanes; λmax, nm
(ε, M−1 cm−1)): 416 (10200), 584 (830).
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details for reactions shown in Scheme 3 and
attempted catalytic HAT reactions, UV−visible spectra, and
crystallographic data for complexes 3, 4, 5, 5a, 6, 7, 8, 9. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Preparation of CpCr[(XylNCMe)2CH][OC(Me)3] (6). Com-
pound 2 (53.2 mg, 0.116 mmol) was placed in a Schlenk flask,
followed by the addition of Et2O (7 mL), and KOtBu (15.2 mg, 0.135
Corresponding Author
*E-mail: kevin.m.smith@ubc.ca.
698
dx.doi.org/10.1021/ic202233f | Inorg. Chem. 2012, 51, 688−700

Inorganic Chemistry
Article
Sadasivam, D. V.; Flowers, R. A. II J. Am. Chem. Soc. 2010, 132,
17396−17398.
ACKNOWLEDGMENTS
■
We are grateful to the Natural Sciences and Engineering
Research Council of Canada (NSERC), the Canadian
Foundation of Innovation, and the University of British
Columbia for financial support. K.M.S. thanks Katherine H.
D. Ballem and Julia L. Conway for initial synthesis of complexes
3, 5, 5a, and 7, Anita Lam for collecting X-ray diffraction data
for complexes 6, 8, and 9, Addison N. Desnoyer for preliminary
studies of 3 with Ph3PI2, and Professor Warren E. Piers for
helpful discussions.
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