Exploring chromium(III)-alkyl bond homolysis with CpCr[(ArNCMe) 2CH](R) complexes

Article abstract of DOI:10.1021/ja1083392

A range of paramagnetic Cr(III) monohydrocarbyl complexes CpCr[(ArNCMe)₂CH](R) (Ar = ortho-disubstituted aryl; R = primary alkyl, trimethylsilylmethyl, benzyl, phenyl, alkenyl, or alkynyl) were synthesized to investigate how varying the steric and electronic properties of the R group affected their propensity for Cr-R bond homolysis. Most complexes were prepared by salt metathesis of known CpCr[(ArNCMe)₂CH](Cl) compounds in Et₂O with commercial RMgCl solutions, although more sterically demanding combinations of Ar and R groups necessitated the use of halide-free MgR₂ reagents and the Cr(III) tosylate or triflate derivatives. Alternative synthetic routes to Cr(III)-R species using the previously reported Cr(II) compounds CpCr[(ArNCMe)₂CH] and sources of R? radicals (e.g., BEt₃ and air) were also explored. The UV-vis spectra of the CpCr[(ArNCMe)₂CH](R) complexes possessed two strong bands with maximum absorbances in the ranges 395-436 nm and 535-582 nm, with the band in the latter range being particularly characteristic of the Cr(III)-R compounds. The Cr-CH₂R bond lengths as determined by single-crystal X-ray diffraction were longer than those in the corresponding Cr-CH₃ complexes, typically falling in the range 2.10 to 2.13 A. The Cr(III) benzyl compounds displayed longer Cr-CH₂Ph distances, while the bond lengths for the alkenyl and alkynyl species were substantially shorter. The rate of Cr-R bond homolysis at room temperature was determined by monitoring the reaction of Cr(III) neopentyl, benzyl, and isobutyl complexes with excess PhSSPh using UV-vis spectroscopy. Although the other primary alkyl, phenyl, and alkenyl compounds did not undergo appreciable homolysis under these conditions, they were cleanly converted to CpCr[(ArNCMe)₂CH](SPh) by photolysis.

Full text of DOI:10.1021/ja1083392

Published on Web 11/11/2010
Exploring Chromium(III)-Alkyl Bond Homolysis with
CpCr[(ArNCMe)2CH](R) Complexes
K. Cory MacLeod,† Julia L. Conway,† Brian O. Patrick,‡ and Kevin M. Smith*,†
Department of Chemistry, UniVersity of British Columbia Okanagan, 3333 UniVersity Way,
Kelowna, BC, Canada V1V 1V7, and Department of Chemistry, UniVersity of British Columbia,
VancouVer, British Columbia, Canada V6T 1Z1
Received September 15, 2010; E-mail: kevin.m.smith@ubc.ca
Abstract: A range of paramagnetic Cr(III) monohydrocarbyl complexes CpCr[(ArNCMe)2CH](R) (Ar ) ortho-
disubstituted aryl; R ) primary alkyl, trimethylsilylmethyl, benzyl, phenyl, alkenyl, or alkynyl) were synthesized
to investigate how varying the steric and electronic properties of the R group affected their propensity for
Cr-R bond homolysis. Most complexes were prepared by salt metathesis of known CpCr[(ArNCMe)2CH](Cl)
compounds in Et2O with commercial RMgCl solutions, although more sterically demanding combinations
of Ar and R groups necessitated the use of halide-free MgR2 reagents and the Cr(III) tosylate or triflate
derivatives. Alternative synthetic routes to Cr(III)-R species using the previously reported Cr(II) compounds
CpCr[(ArNCMe)2CH] and sources of R · radicals (e.g., BEt3 and air) were also explored. The UV-vis spectra
of the CpCr[(ArNCMe)2CH](R) complexes possessed two strong bands with maximum absorbances in the
ranges 395-436 nm and 535-582 nm, with the band in the latter range being particularly characteristic of
the Cr(III)-R compounds. The Cr-CH2R bond lengths as determined by single-crystal X-ray diffraction
were longer than those in the corresponding Cr-CH3 complexes, typically falling in the range 2.10 to 2.13
Å. The Cr(III) benzyl compounds displayed longer Cr-CH2Ph distances, while the bond lengths for the
alkenyl and alkynyl species were substantially shorter. The rate of Cr-R bond homolysis at room temperature
was determined by monitoring the reaction of Cr(III) neopentyl, benzyl, and isobutyl complexes with excess
PhSSPh using UV-vis spectroscopy. Although the other primary alkyl, phenyl, and alkenyl compounds
did not undergo appreciable homolysis under these conditions, they were cleanly converted to
CpCr[(ArNCMe)2CH](SPh) by photolysis.
strated by Vicic,5 Hu,6 and Ca´rdenas7 in nickel-based cross-
coupling catalysts. Somewhat counterintuitively, it is the
Introduction
Alkyl complexes of first-row transition metals have gained
increasing prominence in catalytic carbon-carbon bond forming
reactions. In cross-coupling reactions involving alkyl halide
substrates, catalysts based on cobalt, nickel, and iron display
reactivity complementary to established palladium systems.1 It
has become increasingly apparent that metal-alkyl bond
homolysis to form carbon-based radicals is a critical step in
many of these successful new catalytic cycles.1 Reversible M-R
homolysis has long been recognized as a characteristic reactivity
mode for Co(III)-alkyl complexes,2 and cobalt reagents have
been employed to control radical reactivity as stoichiometric
reagents3 and as catalysts4 for organic synthesis. However,
reversible metal-alkyl bond homolysis has also been demon-
intermediacy of radicals that permit the enantioselective conver-
sion of racemic organic halides to chiral products in Fu’s cross-
coupling reactions with chiral nickel catalysts.8 Even in iron
systems, the reaction of Fe(II) with organic radicals to generate
Fe(III) and carbon-based radicals has been proposed as a key
feature of catalytic C-C bond forming reactions9,10 and as a
means to shuttle between Fe(II)/Fe(0) and Fe(III)/Fe(I) redox
couples.11
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† University of British Columbia Okanagan.
‡ University of British Columbia, Vancouver.
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J. AM. CHEM. SOC. 2010, 132, 17325–17334 17325

A R T I C L E S
MacLeod et al.
Scheme 1. Reversible Metal-Alkyl Bond Homolysis
Scheme 2. Synthesis of Aqueous Cr(III) Alkyl Compounds via
Radical C-H Activation
relatively invariant with the identity of the organic radical.22
High-pressure kinetic studies indicated that the radical trapping
rate is hindered by dissociation of H2O from the [Cr(H2O)6]2+
dication,22 even though water exchange in Cr(II)(aq) is among
the very fastest rates for any [M(H2O)n]m+ aqueous transition
metal complex.23 The efficient trapping of R · by Cr(II) also
underpins the Cr-mediated coupling of organic halides and
aldehydes.24 The reaction of Cr(II) and R · is only rendered
reversible through adverse steric interactions and/or significant
electronic stabilization of the resulting organic radical.20,25
The ability of metal alkyl complexes to both generate and
trap carbon-based radicals is also the foundation of organome-
tallic-mediated radical polymerization (OMRP),12 where the low
concentration of R · in solution is maintained by the equilibrium
shown in Scheme 1a. Although first developed for cobalt
reagents,13 OMRP has since been demonstrated using other first-
row transition metals including iron14 and vanadium.15 We
recently reported the use of a well-defined Cr(III) alkyl complex
to control the radical polymerization of vinyl acetate.16 The
effectiveness of the OMRP reagent was found to depend on
the identity of the alkyl group. The methyl complex,
CpCr[(XylNCMe)2CH](CH3) (1) (Xyl ) xylyl, 2,6-Me2C6H3),
that we had previously synthesized in the investigation of the
single-electron oxidative addition of iodomethane with
CpCr[(XylNCMe)2CH]17 was found to generate only small
amounts of poly(vinyl acetate) with poor polydispersity after
48 h in neat vinyl acetate at room temp. In contrast, the
corresponding neopentyl complex CpCr[(XylNCMe)2CH]-
(CH2CMe3) (2) served both to initiate and to control the radical
polymerization of vinyl acetate.16
We are interested in using well-defined Cr(III) alkyl com-
plexes for controlling radical intermolecular C-C bond forming
reactions, both for OMRP and for synthetic organic applications.
Understanding how varying the identity of the alkyl group
influences its propensity for homolytic Cr-R bond cleavage
would aid in the design of these reagents. The CpCr-
[(ArNCMe)2CH](R) system possesses several attractive features
for this type of investigation (Scheme 1b). The ꢀ-diketiminate
ligand can be systematically modified,26 and the resulting Cr(III)
alkyl complexes are readily recrystallized from hexanes. The
high spin d4, Cr(II) complexes CpCr[(ArNCMe)2CH] can be
independently synthesized;17,27,28 are monomeric and stable as
solids and in solution under anhydrous, anaerobic conditions;
and are coordinatively unsaturated. The latter factor ensures that
no ligand dissociation is required prior to radical trapping, unlike
some Co(II) complexes29 or the Cr(II)(aq) species mentioned
above.22 Computational studies indicate that the barrier for
Cr(III)-R formation from CpCr[(ArNCMe)2CH] and alkyl
radicals is only ∼1 kcal/mol.16 The use of monoalkyl Cr(III)
complexes also avoids complications due to intramolecular alkyl
group reactivity observed for known bis- or tris-alkyl Cr(III)
systems. For instance, Sneeden noted that the trend for thermal
stabilities of CrR3(THF)n complexes (aryl > allyl > benzyl >
alkyl > alkenyl) did not correlate with the relative stabilities of
the corresponding organic radicals.18 Similarly, Cp*Cr(L)(R)2
These observations are consistent with previously established
organochromium chemistry.18 The ability of aqueous Cr(II) to
trap carbon-based radicals was demonstrated over half a century
ago with the preparation of [(H2O)5Cr(CH2Ph)]2+
.
19 Initially
(aq)
synthesized by single-electron oxidative addition of Cr(II) with
benzyl chloride, a wider range of Cr(III) alkyl species can be
prepared in situ via the radical C-H bond activation process
shown in Scheme 2. Mixtures of Cr(II)(aq) and an excess of the
organic substrate are treated with hydroxyl radicals, generated
20
by the reaction of Cr(II)(aq) with added H2O2 or by pulse
radiolysis.21 The reactive ·OH radical abstracts H · from the
ether or alcohol, and rapid trapping of the resulting organic
radical with Cr(II) forms the observed [(H2O)5Cr(CR2(OR))]2+
(aq)
species. The rate of trapping of R · by Cr(II)(aq) is close to the
diffusion control limit at 2 × 107 to 3 × 108 M-1 s-1 and is
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Exploring Chromium(III)-Alkyl Bond Homolysis
A R T I C L E S
Chart 1
Scheme 3. Synthetic Routes to Cr(III)-R Compounds
prepared from the Cr(III) triflate derivative, as previously
reported for Cr(III) methyl complex 1a.27 Both Cr(III) chloride
and Cr(III) tosylate precursors were used to prepare
CpCr[(ArNCMe)2CH](R) complexes (Ar ) Xyl, Dep, or Mes)
by salt metathesis, as shown in Scheme 3. The more convenient
route is method A: CpCr[(ArNCMe)2CH](Cl) reacted in Et2O
at room temp with commercial solutions of RMgX, followed
by addition of commercial anhydrous 1,4-dioxane, Celite
filtration to remove the precipitated MgX2 salts, and recrystal-
lization of the Cr(III) alkyl products from hexanes at -35 °C.
In cases where the desired combination of R and NAr substit-
uents resulted in significant steric congestion, an alternative
synthetic approach was employed. The Cr(III) tosylate precur-
sors for method B in Scheme 3, 3b (Ar ) Dep) and 3c (Ar )
Mes), were synthesized by treating CpCr[(ArNCMe)2CH](Cl)
with AgOTs as described for 3 (Ar ) Xyl)16 and were air-
stable as crystalline solids. In method B, Cr(III) tosylate complex
3, 3b, or 3c was reacted with halide-free MgR2 reagents to avoid
competing Cr(III)-X formation.33 For example, although
CpCr[(XylNCMe)2CH](CH2SiMe3), 4, (Figure 1) could be
prepared via method A, the reaction proceeded to completion
more quickly using method B.
complexes display distinct decomposition modes depending on
the alkyl substituent for R ) allyl,30 benzyl,31 or trimethylsi-
lylmethyl.32
In this paper, we report the synthesis of the CpCr-
[(XylNCMe)2CH](R) complexes shown in Chart 1. The struc-
tures of all of these complexes were confirmed by single crystal
X-ray diffraction (see Supporting Information for crystal-
lographic details). For specific alkyl complexes, we have also
prepared the analogous compounds bearing ꢀ-diketiminate
ligands with alternative N-aryl substituents. In this paper, these
variants will be designated as shown in Chart 1 for the
previously reported CpCr[(ArNCMe)2CH](Me) complexes: 1a,
Ar ) Dpp (2,6-diisopropylphenyl, 2,6-(Me2CH)2C6H3); 1b, Ar
) Dep (2,6-diethylphenyl, 2,6-(CH3CH2)2C6H3); 1c, Ar ) Mes
(mesityl, 2,4,6-Me3C6H2).17,27 The reaction of these complexes
with excess PhSSPh was monitored using UV-visible spec-
troscopy, permitting the determination of the Cr(III)-R ho-
molysis rates for the more reactive neopentyl and benzyl
compounds. Although no thermal homolysis was measured for
most of the remaining complexes at ambient temperature, the
presence of excess PhSSPh revealed photolytic reactivity that
we had not previously appreciated.
Method C in Scheme 3 takes advantage of the ability of Cr(II)
to trap carbon-based radicals. The application of this route to
synthesize 4 by reacting CpCr[(XylNCMe)2CH] with 1.5 equiv
Results and Discussion
Synthetic Routes to CpCr[(ArNCMe)2CH](R) Complexes.
The new CpCr[(DppNCMe)2CH](R) compounds (R
CH2SiMe3 (4a), CH2CHMe2 (7a), and CH2Ph (10a)) were
)
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K. H. Organometallics 1995, 14, 738–745.
Figure 1. Thermal ellipsoid diagram (50%) of 4. All H atoms and one
half-molecule of hexanes are omitted for clarity.
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A R T I C L E S
MacLeod et al.
Scheme 4. Synthesis of 4 from R-I and Mn (Method C)
Scheme 5. Attempted Synthesis of Cr(III)-CH2CMe2Ph Complexes
of Me3SiCH2I and an excess of Mn is shown in Scheme 4.34
Use of Zn or Mg to reduce the Cr(III) iodide back to the active
Cr(II) form did not lead to clean conversion to 4, as determined
by UV-vis spectroscopic analysis of the crude reaction
mixtures. Manganese powder has found use as a selective
stoichiometric reductant for other reactions of first-row transition
metals with organic substrates, from Fu¨rstner’s catalytic-in-
chromium variant of the Nozaki-Hiyama-Kishi reaction35 to
more recent work with titanocene36 and nickel37 catalysts.
low yields from this reaction demonstrated that the product was
neither the presumed neophyl intermediate CpCr-
[(DppNCMe)2CH](CH2CMe2Ph) (6a) nor the direct result of
ancillary ligand deprotonation, but rather the product of activa-
tion of the THF reaction solvent, 6a′.40 Attempts to prepare the
cyclopentyl analog of 6a′, CpCr[(DppNCMe)2CH](C5H9), by
salt metathesis also gave unanticipated results.41 Subsequent
reactions of Cr(III) tosylate 3 or 3c with halide-free
Mg(CH2CMe2Ph)2 reagents also gave very low yields of
crystalline products. X-ray diffraction revealed these to be Cp-
free metalated Cr[(XylNCMe)2CH](C6H4CMe2CH2) (6′)42 (Fig-
ure 2) and the Cr(II) complex CpCr[(MesNCMe)2CH] (Scheme
5). Presumably there is a mixture of compounds formed for all
of the reactions with preferential recrystallization resulting in
characterization of different complexes as the ꢀ-diketiminate
ligand is changed. No further attempts were made to optimize
the reaction with Mg(CH2CMe2Ph)2 to obtain 6 or any one of
the other products shown in Scheme 5.
Another example of method C is the synthesis of CpCr-
[(XylNCMe)2CH](CH2CH3) (5) shown in eq 1. By UV-vis
spectroscopy, there is no apparent reaction between BEt3 and
CpCr[(XylNCMe)2CH] in Et2O. However, exposure of the dilute
reaction mixture to air results in the rapid consumption of Cr(II)
and the clean formation of 5, as determined by comparison of
the UV-vis spectrum of an authentic sample prepared via
method A. Both trialkylboranes and B-alkylcatecholboranes are
known to release R · when treated with O2 or alkoxy radicals.38
The presence of excess air, BEt3, and oxidized boron byproducts
renders the reaction in eq 1 unsuitable for isolation of the Cr(III)
ethyl complex. However, it does present an intriguing potential
method for the in situ generation of functionalized Cr(III) alkyl
species derived from hydroboration.
Attempted Synthesis of Neophyl Complexes. As noted in our
initial 2004 communication, attempts to prepare CpCr-
[(DppNCMe)2CH](R) complexes more sterically demanding
than methyl complex 1a were unsuccessful.27 Suspecting
possible deprotonation of the ꢀ-diketiminate ligand,39 we tried
the reaction shown in Scheme 5 to identify the major decom-
position product. X-ray diffraction of the crystals isolated in
Gratifyingly, only relatively minor variations of the
Cr-CH2CMe2Ph group were required to produce the desired
Cr(III) alkyl complexes. The Dep (2b) and Mes (2c) substituted
neopentyl compounds were prepared by method B, analogous
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Exploring Chromium(III)-Alkyl Bond Homolysis
A R T I C L E S
Figure 4. Thermal ellipsoid diagram (50%) of 10b. All H atoms are omitted
for clarity.
Figure 2. Thermal ellipsoid diagram (50%) of 6′. Compound 6′ has two
independent molecules in the crystal lattice; only one is shown, and all H
atoms are omitted for clarity.
Figure 3. Thermal ellipsoid diagram (50%) of 9. All H atoms are omitted
for clarity.
Figure 5. Thermal ellipsoid diagram (50%) of 11. All H atoms are omitted
for clarity.
to the recent synthesis of 2.16 Although attempts to prepare the
Dpp neopentyl complex also led only to reduction to Cr(II),
the corresponding isobutyl compound CpCr[(DppNCMe)2CH]-
(CH2CHMe2) (7a) could be isolated via method B. For the Xyl
ꢀ-diketiminate ligand, method A was used to prepare isobutyl
7 and phenethyl 8. Replacing the phenyl and/or methyl
substituents of the neophyl ligand with ꢀ-hydrogens thus appears
to improVe the stability of the Cr(III) alkyl complexes. This
not only underscores the characteristic reluctance of paramag-
netic Cr(III) complexes to engage in ꢀ-hydrogen elimination
reactions18,43 but also indicates the high degree of steric
discrimination of the CpCr[(ArNCMe)2CH] system.16
The benzyl complexes CpCr[(ArNCMe)2CH](CH2Ph) (Ar )
Xyl (10), Dpp (10a), Dep (10b), and Mes (10c)) were prepared
by method A (10, 10b, 10c) or from the Cr(III) triflate precursor
for 10a. The X-ray structure of complex 10b is shown in Figure
4. For both cyanomethyl and benzyl ligands, the substituents
stabilize the ·CH2CN and ·CH2Ph radicals, respectively, which
should result in a weaker Cr-R bond.12a For instance, primary
alkyl [Cr-CH2R]2+(aq) species do not undergo homolysis, while
benzylic [Cr-CH2Ph]2+ do.48
(aq)
Synthesis via Solvent C-H Atom Abstraction. The Cr-neo-
pentyl derivative 2 was found to convert cleanly to the
Cr-benzyl derivative 10 when dissolved in toluene. A similar
reaction was observed when 2 was dissolved in a 10:1 mixture
of benzene/p-xylene to yield CpCr[(XylNCMe)2CH]-
(CH2C6H4Me) (11) as determined by X-ray diffraction (Figure
5). The observed product of the reaction suggested a radical
mechanism by which 2 underwent Cr-C bond homolysis to
generate a neopentyl radical, followed by hydrogen atom
abstraction from the p-xylene to generate a ·CH2C6H4Me radical
that could then be trapped by CpCr[(XylNCMe)2CH], as shown
in Scheme 6. The observed intramolecular (10) and intermo-
Electronic Effects: Cyanomethyl and Benzyl. Although ad-
verse steric interactions are undoubtedly involved in the
homolysis of Cr(III)-alkyl bonds, we also wished to investigate
alkyl group electronic effects. The cyanomethyl compound
CpCr[(XylNCMe)2CH](CH2CN) (9) was readily prepared from
the Cr(III) chloride precursor and in situ generated KCH2CN.44
As shown in Figure 3, the CH2CN ligand is C-bound to the Cr
as a cyanomethyl45 and does not bind through the heteroatom
as a keteniminate.46 This contrasts with the recently reported
enolate complex CpCr[(XylNCMe)2CH][OC(Ph)CH2], which
was found to bond to Cr through the oxygen atom.47
(46) (a) Fulton, J. R.; Sklenak, S.; Bouwkamp, M. W.; Bergman, R. G.
J. Am. Chem. Soc. 2002, 124, 4722–4737. (b) Oertel, A. M.; Ritleng,
V.; Chetcuti, M. J.; Veiros, L. F. J. Am. Chem. Soc. 2010, 132, 13588–
13589.
(43) Theopold, K. H. Acc. Chem. Res. 1990, 23, 263–270.
(44) Culkin, D. A.; Hartwig, J. F. Acc. Chem. Res. 2003, 36, 234–245.
(45) (a) Huber, T. A.; Macartney, D. H.; Baird, M. C. Organometallics
1995, 14, 592–602. (b) Derrah, E. J.; Giesbrecht, K. E.; McDonald,
R.; Rosenberg, L. Organometallics 2008, 27, 5025–5032.
(47) Champouret, Y.; MacLeod, K. C.; Smith, K. M.; Patrick, B. O.; Poli,
R. Organometallics 2010, 29, 3125–3132.
(48) Espenson, J. H. Acc. Chem. Res. 1992, 25, 222–227.
9
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A R T I C L E S
MacLeod et al.
Scheme 6. Reaction of 2 in Toluene or p-Xylene To Produce 10 or
11
Figure 7. Thermal ellipsoid diagram (50%) of 14. All H atoms are omitted
for clarity.
lecular (11) selectivity for benzylic over aromatic C-H bond
activation is consistent with the trend in C-H bond dissociation
energies of H-CH2Ph < H-CH2CMe3 < H-Ph.49,50 A similar
radical solvent C-H activation mechanism may be operative
in the generation of complex 6a′, discussed above. In these
reactions, the reactive ·CH2CMe2R (R ) Me or Ph) radicals
serve the same role as observed for the ·OH radicals in the
synthesis of aqueous Cr(III) alkyl complexes as shown in
Scheme 2.
Phenyl, Alkenyl, and Alkynyl Complexes. The Cr(III) phenyl
complex CpCr[(XylNCMe)2CH](Ph) (12) (Figure 6) was pre-
pared by method B. Compound 12 was found to be remarkably
stable with no decomposition observed by UV-vis after 4 days
at 10-4 M in THF or benzene, and it did not initialize
polymerization of vinyl acetate at 55 °C after 48 h. The alkenyl
complex CpCr[(XylNCMe)2CH](CHdCMe2) (13) and the par-
ent alkynyl complex CpCr[(XylNCMe)2CH](CCH) (14) were
both prepared by method A. The X-ray structure of the Cr-CCH
complex is shown in Figure 7.
UV-visible Spectroscopy. All of the CpCr[(ArNCMe)2CH]R
compounds exhibit two strong absorbance peaks in their
UV-vis spectra with λmax in the ranges 395-436 nm and
535-582 nm (Figure 8). The higher energy peak of the phenyl
(12), alkenyl (13), and alkynyl (14) compounds is shifted slightly
(424-436 nm) compared to the other compounds being reported
(395-420 nm). Interestingly, the complexes without Cr-C
σ-bonds have peaks in the same ranges but with very different
extinction coefficients (ε); the ε values for the Cr(III) chloride,
Figure 8. UV-vis spectra of compounds 2c, 10, 15, and CpCr-
[(XylNCMe)2CH] at 1.1 × 10-4 M in hexanes.
tosylate, and triflate species is larger than the hydrocarbyl
compounds for the peak at ∼400 nm but is significantly smaller
for the lower energy peak at ∼550 nm. As a result, UV-visible
spectroscopy was invaluable as a preliminary characterization
technique for these compounds due to the easily distinguishable
spectra of the starting materials and the products of the alkylation
reactions.
Although the Cr(III) neopentyl compounds 2b and 2c were
purified by recrystallization from hexanes, they were found to
convert to Cr(II) when diluted to concentrations suitable for
UV-vis (10-4 M), as previously noted for 2.16 The stability of
the Cr(III)-R complexes is concentration dependent, since the
equilibrium shown in Scheme 1b favors dissociation with
dilution. In contrast, UV-vis samples of the other Cr(III)-R
compounds in Chart 1 showed no appreciable buildup of Cr(II)
during sample preparation.
Notably, small changes in the UV-vis spectra provide a
qualitative distinction between many of the compounds due to
their differences in color. The Cr(III) chloride, tosylate, and
triflate compounds are green to incident light and orange to
transmitted light, while many of the Cr(III)-R compounds are
purple to both incident and transmitted light. Some exceptions
are the Cr(III)-CH2CN compound which is green to incident
and red to transmitted light and the green-blue Cr(III)-CH2Ph
(49) Blanksby, S. J.; Ellison, G. B. Acc. Chem. Res. 2003, 36, 255–263.
(50) Nonradical C-H activation reactions with transition-metal complexes
usually display a thermodynamic preference for strong C-H bonds;
see: Balcells, D.; Clot, E.; Eisenstein, O. Chem. ReV. 2010, 110, 749–
823.
Figure 6. Thermal ellipsoid diagram (50%) of 12. All H atoms are omitted
for clarity.
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Exploring Chromium(III)-Alkyl Bond Homolysis
A R T I C L E S
Table 1. Bond Lengths and Angles for CpCr[(ArNCMe)2CH](R)
< Dpp). The largest Cr-C-Si angle is also found for 4a, the
complex with the bulkiest Ar substituent. The ethyl complex 5
has a less distorted Cr-C-C angle than 4 but still shows a
significantly larger Cr-R distance compared to the correspond-
ing methyl compound 1. The secondary alkyl complex 6a′ lies
well within the 2.10 to 2.13 Å range. The isobutyl compounds
7 and 7a have relatively long Cr-C bonds, but the Cr-C-C
angles are significantly smaller than that of either the neopentyl
or trimethylsilylmethyl species, perhaps due to the better fit of
the isobutyl group in the cleft defined by the flanking ortho-
disubstituted NAr groups. The anomalous bond lengths of the
Complexes
Ar
R
Cr-C (Å)
Cr-C-E (deg)
2c
4
4a
Mes
Xyl
Dpp
CH2CMe3
CH2SiMe3
CH2SiMe3
2.128(2)
2.0987(14)
2.110(2)
2.118(12)
2.1098(15)
2.1031(17)
2.090(5)
2.104(5)
2.122(4)
2.121(5)
2.125(5)
2.1269(15)
2.1199(15)
2.200(7)
2.107(7)
2.126(2)
2.1526(15)
2.1239(13)
2.144(2)
2.1384(19)
2.131(4)
2.098(3)
2.050(4)
2.038(4)
2.009(5)
134.37(16)
131.48(48)
135.71(11)
134.71(11)
130.31(8)
131.89(9)
127.1(3)
127.4(4)
122.0(3)
126.5(4)
126.6(4)
4b
4c
5
Dep
Mes
Xyl
CH2SiMe3
CH2SiMe3
CH2CH3
6a′
7
Dpp
Xyl
C4H7O
CH2CHMe2
phenethyl compound
8 are attributed to the CpCr-
7a
8a
Dpp
Xyl
CH2CHMe2
CH2CH2Ph
126.15(11)
126.49(11)
114.0(4)
[(ArNCMe)2CH]Cl starting material that it cocrystallized with,
as has previously been observed for this class of compounds.17,47
Although the small Cr-C-C bond angle for 9 indicates that
the cyanomethyl group experiences minimal steric hindrance,
its Cr-C bond length is much longer than that of ethyl
compound 5, presumably due to the electronic stabilization
afforded by the cyano substituent.
119.6(4)
9
10
10a
10b
10c
11
Xyl
Xyl
Dpp
Dep
Mes
Xyl
Xyl
Xyl
CH2CN
CH2Ph
CH2Ph
CH2Ph
CH2Ph
CH2C6H4Me
Ph
114.67(16)
121.73(11)
126.30(9)
121.63(14)
120.66(14)
124.3(3)
124.8(3)
142.1(3)
142.4(3)
176.6(5)
12
13
The most intriguing bond length trends are those involving
the benzyl complexes. The Cr-CH2Ph bonds are all very long
even though their steric constraints are greatly reduced compared
to the neopentyl compounds, due to the ability of the flat phenyl
substituent to rotate away from the Cp and ꢀ-diketiminate
ligands. This is consistent with the weaker Cr-CH2Ph bonds
being attributable to the electronic stabilization of the benzylic
radical. There is also a large variation in the Cr-C bond lengths
as a function of the NAr groups in the CpCr[(ArNCMe)2CH]-
(CH2Ph) compounds, from 2.1239(13) to 2.1526(15) Å. How-
ever, the order of the Cr-C bond lengths (Ar ) Dpp 10a <
Mes 10c < Dep 10b < Xyl 10) does not track with the expected
steric demands of the ortho-disubstituted NAr groups (Xyl ≈
Mes < Dep < Dpp) or match the experimentally determined
homolysis rates (10 ∼ 10c < 10a ≈ 10b) as discussed below.
The stretching of the Cr-R bond appears to be a very soft
deformation mode for the Cr(III) benzyl complexes, with the
observed variations in bond lengths presumably being attribut-
able to crystal packing effects. This matches the previous
computational study of bond homolysis of CpCr[(ArNCMe)2CH]-
[CH(Me)OC(O)Me] and its microscopic reverse, trapping R ·
radicals with CpCr[(ArNCMe)2CH]. The Cr-C bond lengths
at the transition states were calculated to be extremely long at
3.440 and 3.396 Å for Ar ) Xyl and Dpp, respectively,
consistent with the very low barrier for trapping R · with the
Cr(II) compounds.16 The inherent difficulties in correlating bond
lengths from single-crystal X-ray diffraction with observed
reactivity trends due to the relative flatness of the potential
energy surface for Cr-R bond breaking are reminiscent of the
problems faced in assessing structure-activity relationships for
Cr-Cr multiple bonds.51 In each case, the loss of Cr-R or
Cr-Cr bonds is compensated by changes in the electronic
structure at the chromium center that lower the energy of the
unsaturated, high-spin chromium products.52 This type of
CHCMe2
14
Xyl
CCH
a Compound 8 cocrystallized with CpCr[(XylNCMe)2CH]Cl.
compounds which have a distinctive shoulder absorbance in their
spectra at 650 nm (Figure 8). The Cr(III)-CH2SiMe3 com-
pounds are also a slightly different color than the other
Cr(III)-R compounds with a violet to transmitted color and
the alkenyl and alkynyl analogues which are red and orange-
red respectively.
X-ray Crystallography. Single-crystal X-ray diffraction was
the preferred method to unambiguously identify each of the
paramagnetic CpCr[(ArNCMe)2CH](R) complexes. We also
wished to correlate the observed chromium-alkyl homolysis
reactivity described below with any structural changes as the
NAr and R groups were modified. For initiation of OMRP of
vinyl acetate by CpCr[(XylNCMe)2CH](R), we had previously
noted that the relative efficiency of the methyl (1) and neopentyl
(2) compounds tracked with their Cr-R bond lengths of
2.076(2) Å and 2.136(3) Å, respectively.16 The Cr-R bond
lengths and Cr-C-E (E ) C or Si) bond angles for the
CpCr[(ArNCMe)2CH](R) compounds are collected in Table 1,
with two sets of values included for compounds that crystallized
with two independent molecules in the unit cell. Complete
crystallographic information is found in the Supporting Informa-
tion.
For the CpCr[(ArNCMe)2CH](R) complexes, the CrsC bond
lengths generally lie in the range 2.10 to 2.13 Å. The CrsPh
and CrsCHdCMe2 bonds are both shorter, at 2.098(3) Å and
2.044(4) (average) Å for 12 and 13, respectively. At 2.009(5)
Å, the CpCr[(XylNCMe)2CH](CCH) complex has a Crsalkynyl
bond that is shorter than the 2.020(4) Å previously reported for
CpCr[(DppNCMe)2CH](CCPh), consistent with the reduced
steric interactions in 14. The neopentyl complex 2c lies near
the end of the typical range at 2.128(2) Å, which is still shorter
than that observed previously for 2.
The relatively slight variation in the structural parameters for
CpCr[(ArNCMe)2CH](CH2SiMe3) as the NAr substituents are
modified follow the expected trends. The observed order of
Cr-CH2SiMe3 bond lengths (4 < 4c < 4b < 4a) match the
increase in steric demand of the Ar groups (Xyl < Mes < Dep
(51) Horvath, S.; Gorelsky, S. I.; Gambarotta, S.; Korobkov, I. Angew.
Chem., Int. Ed 2008, 47, 9937–9940, and references therein.
(52) Poli, R. Chem. ReV. 1996, 96, 2135–2204.
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A R T I C L E S
MacLeod et al.
Scheme 7. Use of Radical Trap X2 To Quantify Cr(III)-R
Homolysis
Figure 9. Thermal ellipsoid diagram (50%) of 15. All H atoms are omitted
for clarity.
electron correlation problem is highly sensitive to ligand
variation53 and is notoriously difficult to model computation-
ally.54
Table 2. Comparison of Rate Constants for CpCr[(ArNCMe)2CH]R
Bond Homolysis
Exploring Cr(III)-R Homolysis. With a series of Cr(III)-R
compounds in hand we aimed to quantify the rate of homolysis
of the Cr-C bonds. One challenge associated with monitoring
the homolysis reactions is the buildup of the persistent radical
trap Cr(II) throughout the reaction,55 leading to a decrease in
observed bond homolysis, by the equilibrium shown in Scheme
1. The bond dissociation energy for M-R can be determined
using a radical trap that only reacts with R · if the trapping
reactions are performed in the presence of excess reduced metal
complex M.2a Alternatively, k values for the bond homolysis
can be obtained using a radical trap, X2, that consumes both
the organic radical and the reduced metal complex (Scheme
7).20
Rate constant
k (s-1
)
Ar
R
2
7
Xyl
Xyl
Dpp
Xyl
Dpp
Dep
Mes
Xyl
CH2CMe3
CH2CHMe2
CH2CHMe2
CH2Ph
CH2Ph
CH2Ph
3.6(3) × 10-3
∼ 1 × 10-5
7a
10
10a
10b
10c
11
∼ 5 × 10-5
3.2(5) × 10-3
9(2) × 10-3
1.1(7) × 10-2
3.2(4) × 10-3
7.8(7) × 10-3
CH2Ph
CH2C6H4Me
mination of the rate constant (k ) 3.2(5) × 10-3 s-1) for the
bond homolysis of 10 at room temperature. A list of rate
constants for bond homolysis of Cr(III)-R compounds is given
in Table 2.
After consideration of alternative radical traps (see Supporting
Information), PhSSPh was chosen as the X2 source. PhSSPh
serves as an effective single electron oxidant for Cr(II) and other
organochromium species.56 Reaction of CpCr[(XylNCMe)2CH]
with 0.5 equiv of PhSSPh in toluene provided the Cr(III)-SPh
Primary Cr(III)-R pentaaqua complexes lacking stabilizing
substituents are not generally observed to generate R · radicals.48
However, neopentyl compounds of the first row transition metals
have previously been reported to be particularly prone to M-R
homolysis.58 The observed rate constant for CpCr-
[(XylNCMe)2CH](CH2CMe3) is consistent with the efficacy of
2 as an initiator for the OMRP of vinyl acetate.16 The dramatic
decrease from 3.6(3) × 10-3 s-1 to ∼1 × 10-5 s-1 upon
changing the alkyl group from neopentyl 2 to isobutyl 7 attests
to the steric discrimination of CpCr[(XylNCMe)2CH].
The dissociation constant of 3.2(5) × 10-3 s-1 measured for
benzyl complex 10 is comparable to the value of 2.6(2) × 10-3
s-1 determined for [(H2O)5Cr(CH2Ph)]2+ in aqueous solution.59
It is also very close to that of neopentyl compound 2. Despite
this favorable dissociation rate, the relative stability of the benzyl
radical makes 10 a less appealing reagent than 2 for OMRP:
the rate constant for the reaction of PhCH2 · with vinyl acetate
is 15 M-1 s-1, compared to 1.4 × 104 M-1 s-1 for CH3 · with
vinyl acetate.60 Electronic effects on the rate of homolysis were
also evident when comparing the benzyl compound (10) to the
para-methylbenzyl compound (11), exhibiting a 2-fold increase
in the rate constant with the introduction of a p-methyl
substituent.59 There was also a clear trend between the four
compound 15 (Figure 9). Alkyl radicals react with PhSSPh with
57
k ) 1.7 × 105 M-1 s-1
,
and treating a 1 × 10-4 M solution
of CpCr[(XylNCMe)2CH] with 6 equiv of PhSSPh had quan-
titatively converted the Cr(II) to the Cr(III)-SPh 15 in the time
(∼3 min) required to transfer the cell from the glovebox to the
UV-vis spectrophotometer. Compound 15 also has a minimum
absorbance in the UV-vis at 560 nm, where the Cr(III)-R
compounds absorb strongly (Figure 8), allowing relatively low
concentrations of Cr(III)-R to be used in homolysis studies
while still achieving large changes in absorbance and being able
to stay within solubility limits of PhSSPh and compound 15 in
hexanes solutions.
The Cr(III)-CH2Ph compound 10 was found to react cleanly
with 10 equiv of PhSSPh to give a first-order observed rate
constant. Saturation kinetics were observed with a sufficient
increase in the PhSSPh concentration allowing for the deter-
(53) (a) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long,
G. J.; Power, P. P. Science 2005, 310, 844–847. (b) Wolf, R.; Brynda,
M.; Ni, C.; Long, G. J.; Power, P. P. J. Am. Chem. Soc. 2007, 129,
6076–6077. (c) La Macchia, G.; Li Manni, G.; Todorova, T. K.;
Brynda, M.; Aquilante, F.; Roos, B. O.; Gagliardi, L. Inorg. Chem.
2010, 49, 5216–5222.
(58) (a) Tsou, T. T.; Loots, M.; Halpern, J. J. Am. Chem. Soc. 1982, 104,
623–624. (b) Wayland, B. B.; Poszmik, G.; Mukerjee, S. L.; Fryd,
M. J. Am. Chem. Soc. 1994, 116, 7943–7944. (c) Fernandez, I.;
Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Organometallics 2008,
27, 109–118.
(54) (a) Hall, M. B. Polyhedron 1987, 6, 679–684. (b) Siegbahn, P. E. M.
AdV. Chem. Phys. 1996, 93, 333–387.
(55) Daikh, B. E.; Finke, R. G. J. Am. Chem. Soc. 1992, 114, 2938–2943.
(56) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J. Organometallics 1997,
16, 5116–5119, and references cited therein.
(59) Nohr, R. S.; Espenson, J. H. J. Am. Chem. Soc. 1975, 97, 3392–3396.
(60) Fischer, H.; Radom, L. Angew. Chem., Int. Ed. 2001, 40, 1340–1371.
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Exploring Chromium(III)-Alkyl Bond Homolysis
A R T I C L E S
different ꢀ-diketiminate ligands when comparing the four
Cr(III)-benzyl compounds (10-10c). The Xyl and Mes deriva-
tives had the same rate constants indicating that there was no
effect of the para-methyl groups of the Mes ligand compared
to the Xyl ligand. There was, however, a clear trend in the steric
profile of the ligands, with the bulkier Dep and Dpp ligands
showing a 3-fold increase in the rate constant.
As previously reported, the Cr(III)-Me compound 1 was not
an efficient radical initiator for OMRP of vinyl acetate, giving
9% conversion over 48 h at room temperature when exposed
to ambient lab light.16 When this experiment was repeated with
the exclusion of light, no poly(vinyl acetate) was produced.
Perhaps the most significant consequence of the observed
photolytic activity of the CpCr[(ArNCMe)2CH](R) compounds
is the connection with their sensitivity to air. The
CpCr[(ArNCMe)2CH] Cr(II) complexes are highly air sensitive
as solids and in solution, as are the Cr(III) neopentyl and benzyl
species. In contrast, the Cr(III) methyl and phenyl compounds
are remarkably air stable in the absence of light. Notably, a
10-4 M solution of 1 in hexanes that was open to air was
monitored by UV-vis in the dark for a 3 h period with no
appreciable change in the spectra. In contrast, a similar solution
of 10 decomposes within minutes upon air exposure in the dark.
In the absence of light, the intact Cr(III) CpCr-
[(ArNCMe)2CH](R) complexes do not apparently undergo outer-
sphere oxidation with O2. However, after photolytic Cr-R
homolysis, the O2 reacts very rapidly with both R · and the Cr(II)
product. The similarity in the photolytic reactivity trends for
the Cr(III)-R complexes using either PhSSPh or O2 as the
radical trap is consistent with the mechanism shown in Scheme
7. The Cr(III) phenyl complex 12 is relatively air stable as a
solid under ambient lab light, consistent with the known stability
of rigorously octahedral Cr(III) phenyl compounds bearing
chelating ligands that do not readily dissociate to reactive five-
coordinate intermediates.62
Compounds 1, 4, 5, 8, 9, 12, 13, and 14 were all subjected
to the same reaction conditions but were found to be stable with
respect to bond homolysis. When the stock solution from the
kinetic experiment with compound 4 was left to stand at ambient
conditions in the glovebox, it did however react with the
PhSSPh, to provide the expected Cr(III)-SPh compound (15),
while the sample in the light-protected spectrophotometer
remained unchanged. When the sample from the instrument was
later removed and left to stand in ambient laboratory light it
cleanly converted to 15 (UV-vis). Further experimentation
revealed that all of the CpCr[(ArNCMe)2CH](R) compounds
except 14 are also cleanly converted to 15 when exposed to
fluorescent lab light or sunlight in the presence of excess
PhSSPh.
Reactivity Consequences of Cr(III)-R Photolysis. Photolysis
is a well-known method to induce M-R bond homolysis,2b and
the photochemistry and photophysics of octahedral Cr(III)
coordination complexes have been very well-studied.61 Never-
theless, the photolytic reactivity with PhSSPh was somewhat
surprising since under typical anaerobic conditions the
CpCr[(ArNCMe)2CH(R) complexes display no indication of
being light sensitiVe. Most of the Cr(III) alkyl compounds were
synthesized in moderate to good yields despite the lack of any
precautions to exclude light during their preparation, recrystal-
lization, or subsequent handling. Solutions of the complexes
prepared in the absence of PhSSPh remained unchanged by
UV-vis after prolonged light exposure. Single crystal samples
of the Cr(III) methyl compound 1b suitable for X-ray diffraction
that were stored under N2 in a flame-sealed clear glass ampule
were not visibly altered or degraded after direct exposure to
ambient laboratory light for over 4 years.
Conclusions
Synthetic routes to CpCr[(ArNCMe)2CH](R) complexes have
been described from the corresponding Cr(III) chloride, tosylate,
or triflate precursors. Rate constants for Cr-R bond homolysis
were obtained by monitoring the reaction of the Cr(III) alkyls
with excess PhSSPh by UV-vis spectroscopy. Modifying the
neopentylligandfromthepreviouslyreportedCpCr[(XylNCMe)2CH]-
(CH2CMe3) complex 2 decreases its reactivity: replacing even
one Me group with a H diminishes the rate of alkyl dissociation,
while use of the CH2SiMe3 group effectively shuts down the
thermal homolysis reaction at room temp. The rates for
Cr-CH2Ph homolysis are comparable to those for 2, even
though the benzyl complexes are much easier to prepare and
handle than the reactive neopentyl compounds. The homolysis
rates increased when bulkier Dpp and Dep substituents were
used on the ꢀ-diketiminate ligands, consistent with the impor-
tance of steric repulsion in weakening the Cr(III)-R bonds.
The deceptive lack of apparent photolytic Cr-R homolysis
is presumably due to the remarkable efficiency of CpCr-
[(XylNCMe)2CH] as a trap for carbon-based radicals, allowing
minute quantities of the Cr(II) complex to effectively preclude
bimolecular R · reactions.20 This reactivity mode only became
apparent when the thermally stable CpCr[(XylNCMe)2CH](R)
compounds were exposed to light in the presence of excess
PhSSPh, which consumes the Cr(II) trap as it is generated.
A 10-4 M solution of methyl complex 1 in hexanes with an
excess of PhSSPh was exposed to light g500 nm in a
fluorometer over a 30 min period with no change observed in
the UV-vis spectrum. Upon exposure to light g400 nm
compound 1 immediately began converting to 15. The clean
conversion of 1 to 15 was essentially complete after a total of
5 h of light exposure (kinetic trace available in the Supporting
Information) over a 2 day period; after 94 min of light exposure
the reaction was stored in the absence of light for 24 h giving
identical UV-vis spectra before and after storage, demonstrating
the need for a continual photon source to cause bond homolysis.
This experiment indicated that the higher energy absorbance
band (395-436 nm) in the UV-vis spectrum was responsible
for bond homolysis.
The remaining CpCr[(ArNCMe)2CH](R) complexes displayed
unexpected Cr-R homolysis reactivity when exposed to ambient
light in the presence of a reagent capable of consuming both
R· and the Cr(II) products, such as PhSSPh or O2. These well-
defined Cr(III) compounds can thus generate hydrocarbyl
radicals with a range of inherent reactivity under mild thermal
or photolytic conditions. We are currently exploring the
controlled generation of R · radicals from CpCr[(ArNCMe)2CH]-
(62) (a) Daly, J. J.; Sanz, F.; Sneeden, R. P. A.; Zeiss, H. H. J. Chem.
Soc., Dalton Trans. 1973, 73–76. (b) Sneeden, R. P. A.; Zeiss, H. H.
J. Organomet. Chem. 1973, 47, 125–131. (c) Cotton, F. A.; Mott, G. N.
Organometallics 1982, 1, 38–43.
(61) Kirk, A. D. Chem. ReV. 1999, 99, 1607–1640.
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A R T I C L E S
MacLeod et al.
N, 5.50. Found: C, 71.06; H, 8.11; N, 5.22. UV/vis (hexanes; λmax
,
(R) species for intermolecular and intramolecular carbon-carbon
bond forming reactions.
nm (ε, M-1cm-1)): 412 (6350), 568 (1180).
Method C. CpCr[(XylNCMe)2CH] (102 mg, 0.241 mmol) was
added to a Schlenk flask in an inert atmosphere glovebox, followed
by the addition of THF (10 mL). ICH2SiMe3 (53.0 µL, 0.357 mmol,
1.49 equiv) was added to the solution and stirred for 10 min
followed by the addition of Mn powder (133 mg, 2.42 mmol, 10
equiv). The reaction mixture was stirred for 24 h at room
temperature, at which point the reaction was determined to be
complete by UV-vis spectroscopy. The reaction mixture was then
stirred for an additional 24 h, and the solvent was removed in Vacuo.
The residue was extracted with hexanes (10 mL), filtered through
Celite, and rinsed with hexanes (3 × 5 mL). The violet solution
was concentrated to 3 mL and cooled to -35 °C to yield crystals
of 4 (89.4 mg, 73%) over several days in two crops, purity
confirmed by UV-vis spectroscopy.
Experimental Section
The complete Experimental Section is found in the Supporting
Information. Representative examples of the three procedures
outlined in Scheme 3 are given below.
Method A. CpCr[(XylNCMe)2CH](Cl) (266 mg, 0.581 mmol)
was added to a Schlenk flask in an inert atmosphere glovebox,
followed by the addition of Et2O (20 mL). ClMgCH2CHMe2 (0.32
mL of 2.0 M solution in Et2O, 0.64 mmol, 1.1 equiv) was added
dropwise to the stirring solution. The reaction mixture was stirred
overnight at room temperature in the dark, and 1,4-dioxane (400
µL) was then added and stirred for 0.5 h at which point the solvent
was removed in Vacuo. The residue was extracted with hexanes
(20 mL), filtered through Celite, and rinsed with hexanes (3 × 5
mL). The purple solution was concentrated to 15 mL, filtered, and
cooled to -35 °C to yield crystals of 7 (179 mg, 64%) over several
days in three crops. Anal. Calcd for C30H39N2Cr: C, 75.12; H, 8.20;
Acknowledgment. We are grateful to the Natural Sciences and
Engineering Research Council of Canada (NSERC), the Canadian
Foundation for Innovation, and the University of British Columbia
for financial support. K.M.S. thanks Katherine H. D. Ballem (UPEI)
for the synthesis of complex 6a′ and Dr. James Bailey for assistance
with the fluorometer photolysis kinetic measurements.
N, 5.84. Found: C, 74.87; H, 7.92; N, 5.86. UV/vis (hexanes; λmax
,
nm (ε, M-1cm-1)): 396 (5510), 556 (1630).
Method B. Compound 3 (202 mg, 0.340 mmol) was added to a
Schlenk flask in an inert atmosphere glovebox, followed by the
addition of Et2O (18 mL). Mg(CH2SiMe3)2 ·1.05(1,4-dioxane) (54.6
mg, 0.188 mmol, 0.552 equiv) in Et2O (4 mL) was added dropwise
to the Schlenk flask. The reaction mixture was stirred overnight at
room temperature, the solvent was removed in Vacuo, the residue
was extracted with hexanes and filtered through Celite, and the
solvent was again removed in Vacuo. The violet solid was extracted
with a minimum amount of hexanes (4 mL), filtered, and cooled
to -35 °C to yield black crystals of 4 (150 mg, 86%) over several
days in two crops. Anal. Calcd for C30H41N2SiCr: C, 70.69; H, 8.11;
Supporting Information Available: Complete experimental
details, UV-visible spectra, and crystallographic data for
complexes 2c, 3b, 3c, 4, 4a, 4b, 4c, 5, 6′, 6a′, 7, 7a, 8, 9, 10,
10a, 10b, 10c, 11, 12, 13, 14, and 15. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA1083392
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