Reactivity consequences of steric reduction in cyclopentadienyl chromium β-diketiminate complexes

Article abstract of DOI:10.1021/om900788c

A series of Cr(III) half-sandwich β-diketiminate complexes, CpCr[(ArNCMe)₂CH]X, X = I (2), CH₃ (3), or Cl (4), were prepared. Compared to previously communicated complexes with Ar = 2,6- ⁱPr₂C₆H₃ (Dpp, a), Cr(III) complexes with less sterically demanding ligands such as Ar = 2,6-Me₂C6 H₃ (XyI, b), 2,4,6-Me₃C₆H₂ (Mes, c), or 2,6-Et₂C₆H₃ (Dep, d) were more readily synthesized via salt metathesis reactions. Iodide compounds 2b-d were prepared by oxidation of the corresponding Cr(II) species CpCr[(ArNCMe)₂CH] 1b-d with half an equivalent of iodine. The Cr(III) chloride complexes CpCr[(ArNCMe)₂CH]Cl 4c-d were prepared in a two-step, one-pot reaction from anhydrous CrCl₃. The reaction of MeMgI with either the Cr(III) chloride or iodide precursors yielded the Cr(III) methyl complexes CpCr[(ArNCMe)₂CH](CH₃) 4b-d. All of the paramagnetic Cr(III) complexes were characterized by UV-visible spectroscopy and elemental analysis, and the structures of 2b, 2c, 3b, 3c, 3d, 4c, and 4d were determined by X-ray crystallography. Reaction of CpCr[(ArNCMe)2CH] with iodomethane generates the Cr(III) iodide and Cr(III) methyl complexes. The rate of this single-electron oxidative addition reaction was shown to remain relatively invariant upon changing the β-diketiminate N-aryl substituent.

Full text of DOI:10.1021/om900788c

6798 Organometallics 2009, 28, 6798–6806
DOI: 10.1021/om900788c
Reactivity Consequences of Steric Reduction in Cyclopentadienyl
Chromium β-Diketiminate Complexes
K. Cory MacLeod,† Julia L. Conway,† Liming Tang,‡ Joshua J. Smith,‡ Liam D. Corcoran,‡
Katherine H. D. Ballem,‡ 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 Prince Edward Island, 550 University Avenue,
Charlottetown, PEI, Canada C1A 4P3, and §Department of Chemistry, University of British Columbia,
Vancouver, British Columbia, Canada V6T 1Z1
Received September 9, 2009
A series of Cr(III) half-sandwich β-diketiminate complexes, CpCr[(ArNCMe)2CH]X, X = I (2),
CH3 (3), or Cl (4), were prepared. Compared to previously communicated complexes with Ar =
2,6-iPr2C6H3 (Dpp, a), Cr(III) complexes with less sterically demanding ligands such as Ar = 2,6-
Me2C6H3 (Xyl, b), 2,4,6-Me3C6H2 (Mes, c), or 2,6-Et2C6H3 (Dep, d) were more readily synthesized
via salt metathesis reactions. Iodide compounds 2b-d were prepared by oxidation of the correspond-
ing Cr(II) species CpCr[(ArNCMe)2CH] 1b-d with half an equivalent of iodine. The Cr(III) chloride
complexes CpCr[(ArNCMe)2CH]Cl 4c-d were prepared in a two-step, one-pot reaction from
anhydrous CrCl3. The reaction of MeMgI with either the Cr(III) chloride or iodide precursors
yielded the Cr(III) methyl complexes CpCr[(ArNCMe)2CH](CH3) 4b-d. All of the paramagnetic
Cr(III) complexes were characterized by UV-visible spectroscopy and elemental analysis, and the
structures of 2b, 2c, 3b, 3c, 3d, 4c, and 4d were determined by X-ray crystallography. Reaction of
CpCr[(ArNCMe)2CH] with iodomethane generates the Cr(III) iodide and Cr(III) methyl complexes.
The rate of this single-electron oxidative addition reaction was shown to remain relatively invariant
upon changing the β-diketiminate N-aryl substituent.
Introduction
catalysts continue to be a major focus, for both systems
involving alkyl aluminum cocatalysts6 and well-defined,
single-component species.7 More recent studies have also
examined oligomerization reactions8,9 and the selective tri-
merization10 and tetramerization11 of ethylene. For each of
these cases, systematic variation of the ancillary ligands has
provided crucial information regarding the mechanisms of
these reactions and the nature of the catalytically active
species.
A distinct class of chromium-catalyzed C-C bond forma-
tion is the coupling of organic halides and aldehydes.12 The
activation of organic halides, R-X, with Cr(II) is generally
accepted to proceed via two single-electron oxidative addi-
tion steps, as shown in Scheme 1.13 The key step of this
reaction is the rapid trapping of organic radicals by Cr(II)
to form inert Cr(III) organochromium species.14 In fact,
The renaissance of the β-diketiminate ligand may be
attributed to its appealing tunability of electronic and steric
properties.1 At one extreme in the range of sterics is the
readily prepared (DppNCMe)2CH [Dpp=2,6-(Me2CH)2C6H3]2
and the even more demanding (DppNCtBu)2CH3 or meta-
terphenyl-substituted4 variants. These ligands have been
demonstrated to stabilize first-row transition metal com-
plexes with unusually low coordination numbers and inter-
esting new modes of reactivity.5
Readily modifiable ancillary ligands are critical for under-
standing structure-activity relationships in organometallic
reactivity. For paramagnetic organochromium complexes,
a variety of carbon-carbon bond-forming applications
are of current research interest. Ethylene polymerization
*Corresponding author. E-mail: kevin.m.smith@ubc.ca.
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pubs.acs.org/Organometallics
Published on Web 11/06/2009
2009 American Chemical Society

Article
Organometallics, Vol. 28, No. 23, 2009 6799
Scheme 1
The chemistry in Scheme 1 has acquired unanticipated new
urgency since our initial communication. Radical intermedi-
ates pervade C-C bond-forming reactions catalyzed by first-
row metals.26 These radicals are integral to shuttling between
discrete two-electron redox cycles (Fe),27 intermolecular
addition of radicals to olefins (Co),28 and enantioselective
synthesis from racemic substrates (Ni).29 The relationships
between oxidative addition by single-electron transfer,30
atom transfer radical polymerization (ATRP),31 and orga-
nometallic-mediated radical polymerization (OMRP)32 have
been elucidated.33 If rendered reversible, the two equations
in Scheme 1 are the equilibria responsible for ATRP and
OMRP, respectively.
catalytic amounts of cobalt or nickel salts are routinely
added to generate R from less reactive alkyl15 or aryl16
3
R-X substrates, respectively, which then react with Cr(II) to
give the functional-group-tolerant organochromium(III)
complex. Since the development of Nozaki-Hiyama-Kishi
reactions that are catalytic in chromium,17 several different
classes of chiral ligands have been used to perform the
catalytic reaction asymmetrically.18,19
Improvement of all of these catalytic reactions, as well as
the development of new ones, will be achieved through
synthetic paramagnetic organometallic chemistry. In order
to establish structure-activity relationships for the critical
bond-dissociation energies (BDEs) in Scheme 1, improved
general routes to well-defined Cr(II), Cr(III)-X, and Cr-
(III)-R complexes will be required. We previously reported
the use of CpCr[(DppNCMe)2CH] as a radical trap for the
OMRP of vinyl acetate initiated with V-70.34 With the
correct match of alkyl and ancillary ligand steric and elec-
tronic effects to attenuate the Cr(III)-R BDE, a single-
component OMRP reagent can be synthesized.35 The Cr-
(III)-CH3 compounds reported in this paper provide an
excellent baseline for these ongoing studies, due to both the
minimal steric pressure of the methyl ligand and the relative
We previously reported the synthesis of CpCr[(Dpp-
NCMe)2CH], its reaction with iodomethane, and the inde-
pendent synthesis of the corresponding Cr(III) methyl and
iodo complexes.20 We were interested in determining how
reducing the steric bulk of the β-diketiminate ligand would
influence the reactivity of these complexes, in terms of both
the synthesis and relative stability of the Cr(II) and Cr(III)
complexes, as well as for the rate of iodomethane oxidative
addition. Theopold and co-workers pioneered the recent
revival of β-diketiminate ligand structure-activity studies
with the synthesis of well-defined Cr(II) and Cr(III) com-
plexes for catalytic olefin polymerization.21 Related struc-
ture-activity studies involving systematic variation of
β-diketiminate ligands have been reported for applications
ranging from C-H bond22 and dioxygen activation23 to
catalytic copolymerization of CO2 and epoxides24 and olefin
metathesis.25
instability of the CH3 radical.
3
Results and Discussion
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1a, served as the starting material for the Cr(III) CpCr-
[(DppNCMe)2CH]X complexes via single-electron oxida-
tion reactions.30 Unlike [Cp*Cr(μ-Cl)]2,36 isolable monocy-
clopentadienyl Cr(II) complexes are not accessible from the
direct reaction of NaC5H5 and CrCl2.37 Initially, CpCr-
[(DppNCMe)2CH] was prepared by reacting NaCp with the
Cr(II) bridging-chloro dimer [Cr[(DppNCMe)2CH](μ-Cl)]2
reported by Gibson and co-workers.38
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6800 Organometallics, Vol. 28, No. 23, 2009
MacLeod et al.
Subsequently, a more direct route was developed to pre-
pare CpCr[(ArNCMe)2CH] 1a-d (Ar = Dpp 1a; Ar = Xyl
(2,6-Me2C6H3) 1b; Ar=Mes (2,4,6-Me3C6H2) 1c; Ar=Dep
(2,6-Et2C6H3) 1d), as shown in eq 1. As reported for the
synthesis of 1b in 2008,34 the reaction involves sequential
addition of NaCp, then Li[(ArNCMe)2CH] to CrCl2 sus-
pended in THF. The reaction can be applied to all four
β-diketiminate derivatives 1a-d and gives similar results
using CrCl2(tmeda) or commercial CrCl2, as well as with
isolated NaCp(THF)x or commercial 2.0 M NaCp in THF.
isolated and structurally characterized from the reaction of
Cp2Cr and NdI2 or DyI2.42 Reaction of Li[(DppNC-
Me)2CH] with CrI2 also gives a bridging iodide dimer, as
demonstrated by X-ray crystallography.21f The Cr(II) [Cr-
[(DppNCMe)2CH](μ-I)]2 dimer can be reduced to a β-dike-
timinate Cr(I) species capable of a range of small molecule
activation reactions.21f,44
Synthesis of Chromium(III) Methyl Complexes. Chro-
mium(III) methyl compounds are frequently prepared as
catalyst precursors for olefin polymerization.7,8,21,38 As
shown in eq 3, CpCr[(ArNCMe)2CH](CH3) complexes
(3b-d) can be synthesized by alkylation of the corresponding
Cr(III) iodide (2b-d) or chloride (4b-d) compounds with
MeMgI in Et2O. Addition of 1,4-dioxane prior to workup
aids in the removal of the MgX2 byproducts of the salt
metathesis reactions. All of these methyl compounds are
highly soluble in nonpolar solvents, including pentane and
hexanes.
Adding NaCp to CrCl2 generates a mixture containing
Cp2Cr as well as unreacted CrCl2, and yet subsequent addition
of Li[(ArNCMe)2CH] gives the mono-Cp Cr(II) complexes in
good yields. The apparent cyclopentadienylide transfer from
Cp2Cr involved in this synthesis is presumably due in part to
the steric bulk imparted by the 2,6-R2 substituents of the
β-diketiminate ligands, which would disfavor the formation
of Cr[(ArNCMe)2CH]2 complexes. The parent Cr[(PhNC-
Me)2CH]2 was prepared by Theopold and co-workers by salt
metathesis of CrCl2 and the phenyl-substituted β-diketiminate
ligand,21a while M[(XylNCMe)2CH]2 complexes can be gener-
ated for other first-row transition metals.5a In contrast, the very
bulky (DppNCtBu)2CH ligand can be used to prepare three-
coordinate Cr[(DppNCtBu)2CH]X complexes.5b
Related cyclopentadienylide displacement reactions from
chromocene have previously been reported. Jonas described
the reaction of LiC6H4CH2NMe2 (prepared by directed
ortho-metalation of N,N-dimethylbenzylamine) with Cp2Cr
to give CpCr(C6H4CH2NMe2),39a which was shown to be a
monomeric complex by X-ray diffraction.39b Other 14-elec-
tron, high-spin, monocyclopentadienyl Cr(II) d4 complexes
have also been prepared from Cp2Cr.40
The ease of synthesis of CpCr[(ArNCMe)2CH](CH2R)
complexes with Grignard reagents appears to depend on
the degree of steric congestion in the Cr(III) alkyl com-
pounds. For Ar = Dpp, the Cr(III) methyl complex was most
readily prepared from a Cr(III) triflate precursor. For the
smaller β-diketiminate ligands where Ar = Xyl, Mes, or Dep,
complexes 3b-d can be obtained from the corresponding
Cr(III) halides.
Synthesis of Chromium(III) Chloride Complexes. As in the
synthesis of 3b-d, the salt metathesis reaction to prepare the
Cr(III) chloride complexes 4b-d also proceeds more readily
with the smaller β-diketiminate ligands.45 A similar trend
was recently reported by Jin and co-workers, who prepared
CpCr[(PhNCMe)2CH]Cl by reaction of Li[(PhNCMe)2CH]
with CpCrCl2(THF),46 while attempts to prepare CpCr-
[(DppNCMe)2CH]Cl by this procedure were unsuccessful.20,46
Interestingly, CpCr[(PhNCMe)2CH]Cl46 and related CpCr-
(LX)Cl and Cp*Cr(LX)Cl complexes47 generate efficient ethy-
lene polymerization catalysts when activated with relatively
small amounts of AlR3 cocatalysts.
Synthesis of Chromium(III) Iodo Complexes. We previously
communicated the use of PbX2 (X = Cl, Br, I)41 to oxidize
CpCr[(DppNCMe)2CH] to the corresponding Cr(III) ha-
lides.20 Synthesis of the CpCr[(ArNCMe)2CH]I complexes
(2b-d) by single-electron oxidation is more conveniently
achieved using 0.5 equiv of I2 in Et2O (eq 2). The isolated
Cr(III) iodide complexes are air stable as crystalline solids.
Related iodide species have been recently reported for
several Cr(II) complexes.21f,42,43 Dimeric [CpCr(μ-I)]2 was
As shown in eq 4, Cr(III) chloride complexes 4b-d are
prepared in a two-step, one-pot reaction starting from
(39) (a) Jonas, K. Angew. Chem., Int. Ed. Engl. 1985, 24, 295–311.
(b) Angermund, K.; Claus, K. H.; Goddard, R.; Kr€uger, C. Angew. Chem.,
Int. Ed. Engl. 1985, 24, 237–247.
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(45) A related Cr(III) chloride complex with the smallest β-diketimi-
nate ligand, Cp*Cr[(HNCMe)2CH]Cl, was prepared 20 years ago by
Theopold and co-workers from the reaction of [Cp*Cr(CH3)(μ-Cl)]2
with NCMe: Richeson, D. S.; Mitchell, J. F.; Theopold, K. H. J. Am.
Chem. Soc. 1989, 8, 2570–2577.
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18, 529–533. (b) Abernethy, C. D.; Clyburne, J. A. C.; Cowley, A. H.; Jones,
R. A. J. Am. Chem. Soc. 1999, 121, 2329–2330.
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(43) (a) Monillas, W. H.; Yap, G. P. A.; Theopold, K. H. J. Chem.
Crystallogr. 2009, 39, 73–77. (b) Monillas, W. H.; Yap, G. P. A.; Theopold,
K. H. J. Chem. Crystallogr. 2009, 39, 377–379. (c) Monillas, W. H.; Yap,
G. P. A.; Theopold, K. H. J. Chem. Crystallogr. 2009, 39, 842–845.
(46) Huang, Y.-B.; Jin, G.-X. Dalton Trans. 2009, 767–769.
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Article
Organometallics, Vol. 28, No. 23, 2009 6801
anhydrous CrCl3 suspended in THF, as described previously
for complex 4b.35 Treatment with Li[(ArNCMe)2CH] gen-
erates red-brown solutions, presumably containing Cr-
[(ArNCMe)2CH]Cl2(THF)n complexes analogous to those
previously reported by Gibson38 and Theopold.21 Reaction
of these solutions in situ with NaCp leads to the desired
CpCr[(ArNCMe)2CH]Cl complexes 4b-d, which can be
recrystallized from hexanes/CH2Cl2. While the intermediate
species are air-sensitive, the anhydrous CrCl3 precursor and
complexes 4b-d are air-stable as solids. Like the Cr(III)
iodide compounds 2b-d, the Cr(III) chloride complexes
4b-d are useful precursors for the synthesis of the Cr(III)
methyl species 3b-d (eq 3).
X-ray Crystal Structures of Cr(III) Methyl and Halide
Complexes. Single-crystal X-ray diffraction was an invalu-
able technique for confirming the identities of the new
Cr(III) complexes. Crystallographic data for complexes 2b,
c, 3b-d, and 4c,d are shown in Tables 1 and 2. The
Cr-X bond lengths for CpCr[(ArNCMe)2CH]X (X = I
(2a-c), CH3 (3a-d), or I (4a-d)) are shown in Table 3.
The molecular structures of the complexes are shown in
Figures 1-7 (CNT denotes the centroid of the cyclopenta-
dienyl ring).
Table 1. Crystal Data and Refinement Parameters for X-ray
Structures of 2b, 2c, and 3b
2b
2c
3b
formula
fw
cryst color, habit
cryst dimens, mm
C26H30N2CrI C28H34N2CrI C27H33N2Cr
549.42 577.47 437.55
black, prism black, block black, irregular
0.50 ꢀ 0.40 ꢀ 0.35 ꢀ 0.25 ꢀ 0.40 ꢀ 0.12 ꢀ
0.20
monoclinic
P21/n
0.20 0.08
orthorhombic triclinic
cryst syst
space group
Pnma
P1
˚
a, A
8.2236(4)
22.710(1)
12.8442(7)
90.0
97.115(4)
90.0
2380.3(2)
4
1.533
1108.00
1.793
620, 8
14.758(1)
21.794(2)
7.9989(7)
90.0
90.0
90.0
2572.7(4)
4
1.491
1172.00
16.62
8.055(1)
11.195(2)
14.967(2)
102.143(8)
103.662(7)
110.500(6)
1163.0(3)
2
1.249
466.00
5.07
620,16
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
Dcalc, g/cm3
F000
μ(Mo KR), cm-1
data images (no., t/s)
2θmax, deg
580, 8
55.0
55.1
24 822
55.1
12 272
4406, 0.028
reflns measd
unique reflns, Rint
absorp, Tmin, Tmax
25 524
2924, 0.013
0.463, 0.699 0.371, 0.717 0.805, 0.960
4706, 0.016
obsd data (I>2.00σ(I)) 4150
no. params 297
2618
154
3548
294
The reactivity of the Cr(III) methyl and halide complexes
was expected to be related to their Cr-X bond dissociation
energies.33 Comparing the structural data for the current
CpCr[(ArNCMe)2CH]X complexes and those previously
reported20 was therefore of interest to obtain structural
evidence for the influence of the β-diketiminate aryl sub-
stituent on the Cr-X bonds. As shown in Table 3, no general
trend in Cr-X bond lengths with varying Ar substituents
was evident. The Cr-CH3 distances are close to that re-
R1, wR2 (F2, all data) 0.037, 0.075 0.028, 0.059 0.056, 0.096
R1, wR2 (F, I>
2.00σ(I))
0.031, 0.073 0.023, 0.057 0.039, 0.090
goodness of fit
max., min. peak3,
1.19
0.64, -0.42
1.06
0.39, -0.50
1.04
0.32, -0.27
-
˚
e /A
3
observed bond lengths are actually more sensitive than either
the electron-density distribution or the thermal para-
meters.50 If present, the amount of the putative 2d cocrys-
tallized impurity in 3d would have to be very small, since the
observed bond length would be weighted by the vastly
greater scattering power of the iodide ligand over that of
the methyl group, as shown by Parkin for tris(3-tert-butyl-
pyrazolyl)hydroboratozinc complexes with cocrystallized I
and CH3 ligands.50a Indeed, our attempts to model a residual
electron density peak beyond the methyl group with a very
low occupancy (around 1%) of an iodide atom did result in
48a
˚
ported for [CpCr(μ-Cl)(CH3)]2 (2.073(3) A),
(PMe3)(CH3)2 (2.067(5) A),
Cp*Cr-
48b
˚
and other neutral Cr(III)
methyl half-sandwich complexes.48c As expected, the
Cr-CH3 bond lengths in 3a-d were longer than in cationic
Cr(III) half-sandwich complexes49 or in neutral38 or catio-
nic21c β-diketiminate Cr(III) methyl compounds with coor-
dination numbers of 5 or less. The overall geometries of the
CpCr[(ArNCMe)2CH]X complexes were relatively constant,
as shown by the selected bond lengths and angles listed in the
captions of Figures 1-7. The only notable difference was the
distance from the Cr to the centroid of the cyclopentadienyl
ligand, Cr-CNT, which was longer for the methyl com-
˚
the contraction of the Cr-CH3 distance to 2.078 A.
Although we were unable to quantitatively model such a
small amount of the presumed impurity with confidence, we
currently consider this to be the most plausible explanation
of the anomalous Cr-CH3 bond length in 3d. In any event,
being cognizant of the potential for impurity cocrystalliza-
tion is especially critical for paramagnetic organometallic
complexes. While the remarkable crystallinity engendered by
the cyclopentadienyl and ortho-disubstituted N-aryl β-dike-
timinate ligands indisputably aids in the synthesis and
structural characterization of CpCr[(ArNCMe)2CH]X com-
plexes, these same properties also promote insidious cocrys-
tallization problems.
˚
plexes (1.952 to 1.967 A for 3b-d) than for the iodide or
˚
chloride compounds (1.901 to 1.911 A for 2b,c and 4c,d),
presumably resulting from the increased electron-donating
ability of the alkyl ligand over that of the halides.
The anomalously long Cr-CH3 distance in 3d needs to be
accounted for. The thermal parameters and electron density
distribution of the methyl carbon atom in 3d do not appear to
be unusual. One possible explanation is that 3d, which was
synthesized from 2d and MeMgI, contains a small amount of
CpCr[(DepNCMe)2CH]I as a cocrystallized impurity. Par-
kin and co-workers have demonstrated that in such cases the
Reactivity with Iodomethane. The ability of Cr(II) to
undergo single-electron oxidative addition reactions with
organic halides has been known since the reaction of
[Cr(H2O)6]2þ with benzyl chloride was found to generate
[Cr(H2O)5Cl]2þ and [Cr(H2O)5(CH2Ph)]2þ in aqueous solu-
tion.51 The mechanism shown in Scheme 1 was initially
(48) (a) Richeson, D. S.; Hsu, S.-W.; Fredd, N. H.; Van Duyne, G.;
Theopold, K. H. J. Am. Chem. Soc. 1986, 108, 8273–8274. (b) Grohmann,
€
A.; Kohler, F. H.; M€uller, G.; Zeh, H. Chem. Ber. 1989, 122, 897–899.
€
€
(c) Dohring, A.; Gohre, J.; Jolly, P. W.; Kryger, B.; Rust, J.; Verhovnik,
G. P. J. Organometallics 2000, 19, 388–402.
(49) Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.;
Theopold, K. H. J. Am. Chem. Soc. 1991, 113, 893–902.
(50) (a) Yoon, K.; Parkin, G. J. Am. Chem. Soc. 1991, 113, 8414–
8418. (b) Parkin, G. Acc. Chem. Res. 1992, 25, 455–460.
(51) Anet, F. A. L.; Leblanc, E. J. Am. Chem. Soc. 1957, 79, 2649–
2560.

6802 Organometallics, Vol. 28, No. 23, 2009
Table 2. Crystal Data and Refinement Parameters for X-ray Structures of 3c, 3d, 4c, and 4d
MacLeod et al.
3c
3d
4c
4d
formula
fw
C29H37N2Cr
465.61
C31H41N2Cr
493.66
C28H34N2CrCl
486.02
C30H38N2CrCl
514.07
cryst color, habit
cryst dimens, mm
cryst syst
black, plate
0.05 ꢀ 0.30 ꢀ 0.50
triclinic
Ph1
9.4802(10)
11.9897(14)
12.2729(14)
70.025(5)
79.696(5)
77.347(5)
1271.0(2)
2
1.217
498.00
4.68
1694, 7
55.0
21 914
black, prism
0.30 ꢀ 0.50 ꢀ 0.50
triclinic
black, tablet
0.20 ꢀ 0.40 ꢀ 0.44
orthorhombic
Pca21
14.3818(10)
15.2520(10)
25.8696(19)
90.0
90.0
90.0
5674.5(7)
8
1.138
green, needle
0.04 ꢀ 0.15 ꢀ 0.30
triclinic
space group
˚
P1
P1
a, A
10.0044(7)
12.3899(7)
13.0646(9)
62.733(1)
81.202(2)
70.457(2)
1356.5(2)
2
1.209
530.00
4.42
2354, 5
8.0490(18)
11.214(3)
15.462(4)
81.291(7)
86.346(8)
74.509(8)
1329.0(6)
2
1.285
546.00
5.52
941, 20
˚
b, A
˚
c, A
R, deg
β, deg
γ, deg
3
˚
V, A
Z
Dcalc, g/cm3
F000
2056.00
5.13
1157, 5
56.0
57 907
μ(Mo KR), cm-1
data images (no., t/s)
2θmax
55.7
28 265
45.6
10 278
reflns measd
unique reflns, Rint
absorp, Tmin, Tmax
obsd data (I > 2.00σ(I))
no. params
5785, 0.026
0.894, 0.977
5022
5712, 0.024
0.804, 0.876
4613
13 222, 0.042
0.809, 0.902
10 409
3437, 0.060
0.771, 0.978
7267
296
326
593
317
R1, wR2 (F2, all data)
R1, wR2 (F, I > 2.00σ(I))
goodness of fit
0.043, 0.094
0.035, 0.088
1.06
0.064, 0.135
0.049, 0.126
1.04
0.054, 0.090
0.038, 0.084
1.00
0.090, 0.144
0.058, 0.130
1.04
3
-
3
˚
max., min. peak , e /A
0.37, -0.30
0.99, -0.46
0.29, -0.32
0.49, -0.48
Figure 1. Thermal ellipsoid diagram (50%) of 2b. Selected bond
˚
Figure 3. Thermal ellipsoid diagram (50%) of 3b. Selected bond
˚
lengths (A): Cr(1)-N(1), 2.017(2); Cr(1)-N(2), 2.021(2); Cr-
lengths (A): Cr(1)-N(1), 2.0246(17); Cr(1)-N(2), 2.0250(17);
(1)-CNT, 1.908; Cr(1)-I(1), 2.6933(5). Selected bond angles
(deg): N(1)-Cr(1)-N(2), 90.09(9); N(1)-Cr(1)-I(12),
95.65(7); N(2)-Cr(1)-I(1), 95.53(7); CNT-Cr(1)-I(1), 117.40.
Cr(1)-CNT, 1.952; Cr(1)-C(27), 2.076(2). Selected bond an-
gles (deg): N(1)-Cr(1)-N(2), 89.96(7); N(1)-Cr(1)-C(27),
94.25(9); N(2)-Cr(1)-C(27), 94.35(9); CNT-Cr(1)-C(27),
118.10.
Figure 2. Thermal ellipsoid diagram (50%) of 2c. Selected bond
˚
lengths (A): Cr(1)-N(1), 2.0277(15); Cr(1)-CNT, 1.905; Cr-
Figure 4. Thermal ellipsoid diagram (50%) of 3c. Selected bond
˚
(1)-I(1), 2.6941(5). Selected bond angles (deg): N(1)-Cr(1)-N-
(1*), 91.54(9); N(1)-Cr(1)-I(1), 96.51(5); CNT-Cr(1)-I(1),
117.06.
lengths (A): Cr(1)-N(1), 2.0245(13); Cr(1)-N(2), 2.0231(13);
Cr(1)-CNT, 1.967; Cr(1)-C(29), 2.0645(17). Selected bond
angles (deg): N(1)-Cr(1)-N(2), 90.73(5); N(1)-Cr(1)-C(29),
93.98(6); N(2)-Cr(1)-C(29), 93.98(6); CNT-Cr(1)-C(29),
117.72.
established with the Cr(II) aqueous system, where rate
constants for the trapping of carbon-based radicals with
[Cr(H2O)6]2þ were shown to be remarkably rapid despite
the need to displace a bound water molecule.14 The overall
rate of the reaction depended on the initial halogen atom
abstraction step and showed the usual trends in terms of
halide (increasing rate with Cl < Br < I) and alkyl
(increasing rate with primary < secondary < tertiary) upon

Article
Organometallics, Vol. 28, No. 23, 2009 6803
˚
Table 3. Comparison of Cr-X Bond Lengths (A) in CpCr-
[(ArNCMe)2CH]X Complexes (X = I (2a-c), CH3 (3a-d), or
Cl (4a-d)
Cr-X
Ar = Dpp
Ar = Xyl
Ar = Mes
Ar = Dep
Cr-I
2a 2.6813(5)a 2b 2.6933(5) 2c 2.6941(5)
Cr-CH3 3a 2.071(2)a
Cr-Cl
4a 2.292(2)a
3b 2.076(2) 3c 2.0645(17) 3d 2.112(3)
4b 2.308(2)b 4c 2.3090(7)
4d 2.2792(11)
a Ref 20. b Ref 34.
ducted in coordinating solvents such as DMF and requires a
cobalt catalyst to activate the organic halide.54 These require-
ments complicate the process of screening ligand reactivity
effects, particularly since any added ligands could potentially
bind to either transition metal present.55 Well-defined organo-
metallic chromium(II) complexes capable of single-electron
oxidative addition of organic halides typically lead to the
Cr(III) halide products but not the more reactive Cr(III) alkyl
species.17,21b,40a,56
Figure 5. Thermal ellipsoid diagram (50%) of 3d. Selected bond
˚
lengths (A): Cr(1)-N(1), 2.0287(19); Cr(1)-N(2), 2.017(2); Cr-
(1)-CNT, 1.965; Cr(1)-C(31), 2.112(3). Selected bond angles
(deg): N(1)-Cr(1)-N(2), 90.20(8); N(1)-Cr(1)-C(31),
93.65(9); N(2)-Cr(1)-C(31), 94.73(9); CNT-Cr(1)-C(31),
118.64.
The reaction shown in eq 5 addresses several of theses
issues. As discussed above, both the reactant and the pro-
ducts are stable and can be independently synthesized.
Complexes 1a-d do not have any additional ligands that
require displacement prior to reaction, and comparison of
the X-ray crystal structures of reactants and products sug-
gests that only minimal reorganization at the Cr center
would be required for the reaction.53b The UV-vis spectra
of the Cr(II), the Cr(III) iodide, and Cr(III) methyl species
are all readily distinguishable, with the peak around 530 nm
for the methyl complexes 3a-d being particularly distinctive.
As previously reported for 1a,20 complexes 1b-d react
with iodomethane in noncoordinating solvents without any
additional cobalt reagent being required to give UV-vis
spectra identical to 1:1 mixtures of the corresponding Cr(III)
iodide and Cr(III) methyl complexes. The ability of com-
plexes 1a-d to successfully trap organic radicals is likely
at least partially due to the stability of Cr(III) CpCrLX2
half-sandwich complexes.57,58 The rates of iodomethane
Figure 6. Thermal ellipsoid diagram (50%) of 4c (one of two
independent molecules in unit cell shown). Selected bond
lengths (A): Cr(1)-N(1), 2.023(2); Cr(1)-N(2), 2.015(2); Cr-
(1)-CNT, 1.911; Cr(1)-Cl(1), 2.3090(7). Selected bond angles
(deg): N(1)-Cr(1)-N(2), 90.65(8); N(1)-Cr(1)-Cl(1), 92.97(6);
N(2)-Cr(1)-Cl(1), 94.46(6); CNT-Cr(1)-Cl(1), 102.72.
˚
(54) Wessjohann, L. A.; Schmidt, G.; Schrekker, H. S. Tetrahedron
2008, 64, 2134–2142.
(55) The potential for ligand scrambling between Cr and Mn or Ni
has been discussed by Cozzi and Kishi, respectively: (a) Bandini, M.;
Cozzi, P. G.; Umani-Ronchi, A. Chem. Commun. 2002, 919–927.
(b) Namba, K.; Cui, S.; Wang, J.; Kishi, Y. Org. Lett. 2005, 7, 5417–5419.
(c) Namba, K.; Wang, J.; Cui, S.; Kishi, Y. Org. Lett. 2005, 7, 5421–5424.
(56) (a) Fischer, E. O.; Ulm, K.; Kuzel, P. Z. Anorg. Allg. Chem. 1963,
319, 253–265. (b) Hermes, A. R.; Morris, R. J.; Girolami, G. S. Organome-
tallics 1988, 7, 2372–2379. (c) Nefedov, S. E.; Pasynskii, A. A.; Eremenko, I.
L.; Orazsakhatov, B.; Ellert, O. G.; Novotortsev, V. M.; Katser, S. B.;
Antsyshkina, A. S.; Porai-Koshits, M. A. J. Organomet. Chem. 1988,
345, 97–104. (d) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J.; Young, V. G.
Jr. J. Chem. Soc., Dalton Trans. 1999, 147–154.
(57) (a) Poli, R. Chem. Rev. 1996, 96, 2135–2204. (b) Heintz, R. A.;
Leelasubcharoen, S.; Liable-Sands, L. M.; Rheingold, A. L.; Theopold, K. H.
Organometallics 1998, 17, 5477–5485, and references therein. (c) Gallant,
A. J.; Smith, K. M.; Patrick, B. O. Chem. Commun. 2002, 2914–2915.
(58) The stabilizing effect provided by the Cp ancillary ligand was
demonstrated by Fryzuk and co-workers in ref 56d with the reactivity of
CrR[N(SiMe2CH2PPh2)2] with benzyl chloride. The square-planar Cr-
(II) alkyl with R = CH2SiMe3 reacted with PhCH2Cl to give the
Cr(R)(Cl)[N(SiMe2CH2PPh2)2] complex only, while the compound with
R=Cp gave a mixture of both CpCr(III) chloride and CpCr(III) benzyl
complexes.
Figure 7. Thermal ellipsoid diagram (50%) of 4d. Selected bond
˚
lengths (A): Cr(1)-N(1), 2.016(3); Cr(1)-N(2), 2.023(2); Cr(1)-
CNT, 1.901; Cr(1)-Cl(1), 2.2972(11). Selected bond angles (deg):
N(1)-Cr(1)-N(2), 90.29(12); N(1)-Cr(1)-Cl(1), 93.54(9); N-
(2)-Cr(1)-Cl(1), 95.03(9); CNT-Cr(1)-Cl(1), 119.90.
varying the R-X substrate.52 Addition of more electron-
donating ligands such as ethylenediamine52 or tetraazamacro-
cycles53 increased the measured rates of the oxidative addition
reaction.
The chromium-mediated coupling of alkyl halides and
aldehydes, the Takai-Utimoto reaction,15 is typically con-
(52) Kochi, J. K.; Powers, J. W. J. Am. Chem. Soc. 1970, 92, 137–146.
(53) (a) Samuels, G. J.; Espenson, J. H. Inorg. Chem. 1979, 18, 2587–
2592. (b) Espenson, J. H. Acc. Chem. Res. 1992, 25, 222–227.

6804 Organometallics, Vol. 28, No. 23, 2009
MacLeod et al.
activation were gauged by reacting 1b-d with at least
10 equiv of MeI and monitoring the growth of the absorption
at 530 nm. The second-order rate constants were calculated
from the slope of plots of kobs vs [MeI] under the pseudo-
first-order conditions. The rate constants obtained were
k = (2.4 ( 0.3) ꢀ 10-2 M-1 s-1 for 1b, k = (2.8 ( 0.2) ꢀ
10-2 M-1 s-1 for 1c, and k=(1.9 ( 0.2) ꢀ 10-2 M-1 s-1 for
1d, compared to the value of k=(2.8 ( 0.3) ꢀ 10-2 M-1 s-1
previously obtained for 1a.20
The similarity of the rate constants for the CpCr-
[(ArNCMe)2CH] complexes was unexpected. It had been
anticipated that varying the ortho substituents of the
β-diketiminate aryl groups would influence the approach
of the MeI to the Cr center in the inner-sphere iodide atom
abstraction, which constitutes the rate-determining step of
and H[(2,6-Et2C6H3NCMe)2CH] were prepared according to the
literature procedure.24 The Cr(II) β-diketiminate compounds 1a,20
1b,34 1c,d,35 and 4b35 were prepared as described in the literature.
UV/vis spectroscopic data were collected on a Varian Cary
100 Bio UV-visible spectrophotometer in pentane or hexanes
solution 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.
Synthesis of CpCr[(2,6-Me2C6H3NCMe)2CH]I (2b). To a
solution of 1b (273 mg, 0.646 mmol) in Et2O was added I2
(81.9 mg, 0.323 mmol) as a solution in 4 mL of Et2O. After
stirring overnight, the solvent was removed in vacuo, the residue
was extracted with Et2O and filtered through Celite, and the
green solution (red to transmitted) was cooled to -30 °C. 2b (198
mg, 55.9%) was isolated in four crops over several days. Anal.
Calcd for C26H30N2CrI: C, 56.84; H, 5.50; N, 5.01. Found: C,
the reaction (Scheme 1). It is possible that the Cr
I
3 3 3
interaction in the transition state is too long for the differ-
ences in Ar groups to have a significant impact on the rate of
the reaction. As noted previously for 1a,20 the rate constants
for 1b-d are comparable to that obtained for the activation
of MeI in aqueous solution for the Cr(II) complex with the
macrocyclic [15]aneN4 ligand, where a value of k=(4.6 ( 1)
ꢀ 10-2 M-1 s-1 was obtained.53a The rate-enhancing effect
of chelating monoanionic N-donor ligands on the oxidative
addition of MeI was also observed in Rh(I) carbonyl com-
plexes.59
56.67; H, 5.75; N, 5.12. Mp: 223-225 °C. UV/vis (pentane; λmax
,
nm (ε, M-1 cm-1)): 431 (7690), 586 (1520).
Synthesis of CpCr[(2,4,6-Me3C6H2NCMe)2CH]I (2c). To
a solution of 1c (176 mg, 0.391 mmol) in Et2O was added I2
(49.5 mg, 0.195 mmol) as a solution in 4 mL of Et2O. After
stirring overnight, the solvent was removed in vacuo, the residue
was extracted with Et2O and filtered through Celite, and the
green solution (red to transmitted) was cooled to -30 °C. 2c
(156 mg, 69.0%) was isolated in two crops over several days.
Anal. Calcd for C28H34N2CrI: C, 58.24; H, 5.93; N, 4.85. Found:
C, 58.01; H, 6.23; N, 5.00. Mp: 232-234 °C. UV/vis (pentane;
Conclusions
λ
max, nm (ε, M-1 cm-1)): 422 (6710), 581 (1550).
Synthesis of CpCr[(2,6-Et2C6H3NCMe)2CH]I (2d). To a solu-
The CpCr[(ArNCMe)2CH] framework appears well suited
to the investigation of single-electron transfer oxidative
addition of organic halides. Well-defined Cr(II) complexes
with tunable ancillary ligands that effectively trap organic
radicals are required for the development of structure-activ-
ity relationships for this reactivity mode, which has applica-
tions for organic synthesis as well as controlled radical poly-
merization. Modifying the β-diketiminate N-aryl groups
from the popular 2,6-diisopropylphenyl group to less steri-
cally demanding substituents did not adversely effect the
relative stability or crystallinity of the Cr(III) products. In
fact, the steric reduction aided in the synthesis of the Cr(III)
chloride and Cr(III) methyl complexes via salt metathesis
reactions. The slight decrease in hexanes solubility also aids
in the recrystallization of these complexes, particularly for
the Cr(III) methyl compounds due to their high solubility in
nonpolar solvents. The CpCr[(ArNCMe)2CH] complexes all
reacted with iodomethane with rate constants ranging from
tion of 1d (104 mg, 0.218 mmol) in 10 mL of Et2O was added I2
(32.4 mg, 0.128 mmol) as a solid. After stirring overnight, the
green solution (red to transmitted) was cooled to -30 °C to yield
2d (77.5 mg, 58.8%). Anal. Calcd for C30H38N2CrI: C, 59.50; H,
6.33; N, 4.63. Found: C, 59.14; H, 6.51; N, 4.75. Mp: 209-211
°C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 423 (7310), 580
(1690).
Synthesis of CpCr[(2,6-Me2C6H3NCMe)2CH]Me (3b). To a
solution of 4b (108 mg, 0.235 mmol) in THF was added MeMgI
(0.15 mL of a 3.0 M solution in Et2O, 0.45 mmol), and the
mixture was left to stir at room temperature overnight. The
solvent was removed in vacuo, the residue was extracted with
pentane and filtered through Celite, and the solvent was again
removed in vacuo. The purple solid was extracted with a mini-
mum amount of pentane, filtered, and cooled to -30 °C over-
night to yield black crystals of 3b (52 mg, 51%). Anal. Calcd for
C27H33N2Cr: C, 74.11; H, 7.60; N, 6.40. Found: C, 73.82; H,
7.89; N, 6.15. Mp: 170-172 °C. UV/vis (pentane; λmax, nm (ε,
M-1 cm-1)): 415 (6070), 543 (2160).
Synthesis of CpCr[(2,4,6-Me3C6H2NCMe)2CH]Me (3c). To a
solution of 4c (105 mg, 0.216 mmol) in 20 mL of Et2O was added
MeMgI (0.080 mL of a 3.0 M solution in Et2O, 0.24 mmol), and
the mixture was left to stir at room temperature overnight. To
the resulting purple solution was added an excess of 1,4-dioxane
(0.20 mL) and allowed to stir for 1 h, at which time the reaction
mixture was filtered through Celite. The solvent was removed
in vacuo, and the residue was extracted with 1 mL of hexanes,
filtered, rinsed with 1 mL of hexanes, and cooled to -35 °C to
yield black crystals of 3c (77.5 mg, 77%) isolated in two crops
over several days. Anal. Calcd for C29H37N2Cr: C, 74.81; H,
8.01; N, 6.02. Found: C, 75.14; H, 8.33; N, 6.18. Mp: 181-183
°C. UV/vis (pentane; λmax, nm (ε, M-1 cm-1)): 406 (6940), 533
(2360).
(1.9 ( 0.2) ꢀ 10-2 to (2.8 ( 0.2) ꢀ 10-2 M-1 s-1
.
Experimental Section
General Considerations. All reactions were carried out under
nitrogen using standard Schlenk and glovebox techniques.
Solvents were dried by using the method of Grubbs.60 Celite
(Aldrich) was dried overnight at 120 °C before being evacuated
and then stored under nitrogen. Iodine was purified by sublima-
tion before use. n-BuLi (1.6 M in hexanes), p-toluenesulfonic
acid monohydrate, NaCp (2.0 M in THF), CrCl3 (anhydrous),
and methylmagnesium iodide (3.0 M in Et2O) were purchased
from Aldrich and used as received. The β-diketiminate ligands
H[(2,6-Me2C6H3NCMe)2CH], H[(2,4,6-Me3C6H2NCMe)2CH],
Synthesis of CpCr[(2,6-Et2C6H3NCMe)2CH]Me (3d). To
a solution of 1d (113 mg, 0.237 mmol) in Et2O was added I2
(35.8 mg, 0.141 mmol) as a solid. After stirring for 2 h, MeMgI
(0.090 mL of a 3.0 M solution in Et2O, 0.27 mmol) was added,
and the solution was left to stir for 1.5 h, at which time an excess
(59) Gaunt, J. A.; Gibson, V. C.; Haynes, A.; Spitzmesser, S. K.;
White, A. J. P.; Williams, D. J. Organometallics 2004, 23, 1015–1023.
(60) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.

Article
Organometallics, Vol. 28, No. 23, 2009 6805
of 1,4-dioxane (0.10 mL) was added. After stirring for 15 min,
the solvent was removed in vacuo, and the residue was extracted
with 15 mL of pentane, filtered through Celite, rinsed with 2 mL
of pentane, and cooled to -30 °C overnight to yield 3d (46.0 mg,
39%). Anal. Calcd for C31H41N2Cr: C, 75.42; H, 8.37; N, 5.67.
Found: C, 75.09; H, 8.76; N, 5.69. Mp: 156-158 °C. UV/vis
(pentane; λmax, nm (ε, M-1 cm-1)): 392 (5000), 521 (1790).
Synthesis of CpCr[(2,4,6-Me3C6H2NCMe)2CH]Cl (4c). A
solution of H[(2,4,6-Me3C6H2NCMe)2CH] (2.35 g, 7.03 mmol)
in 20 mL of THF in a Schlenk flask was cooled to 0 °C in an
ice-water bath. n-BuLi (4.90 mL of 1.6 M solution in hexanes,
7.84 mmol) was added dropwise with stirring. The resulting
yellow solution was allowed to react for 40 min while warming to
room temperature, at which time it was cannulated into a Schlenk
flask containing a suspension of CrCl3 (1.13 g, 7.15 mmol) in THF
(10 mL) and was left to stir at room temperature overnight. NaCp
(3.90 mL of a 2.0 M solution in THF, 7.80 mmol) was added, and
the solution was again left to stir overnight at room temperature.
The solvent was removed in vacuo, and the residue was extracted
with 65 mL of hexanes and 18 mL of dichloromethane, filtered
through Celite, and rinsed with 10 mL of hexanes. The resulting
green solution (orange to transmitted) was concentrated and
cooled to -20 °C to yield dark green crystals of 4c (2.55 g,
74.7%) isolated in four crops over several days. Anal. Calcd for
C28H34N2CrCl: C, 69.19; H, 7.05; N, 5.76. Found: C, 69.17; H,
7.43; N, 5.43. Mp: 221-223 °C. UV/vis (hexanes; λmax, nm (ε, M-1
cm-1)): 419 (8060), 583 (798).
Synthesis of CpCr[(2,6-Et2C6H3NCMe)2CH]Cl (4d). A solu-
tion of H[(2,6-Et2C6H3NCMe)2CH] (3.65 g, 10.1 mmol) in
60 mL of THF in a Schlenk flask was cooled to 0 °C in an
ice-water bath. n-BuLi (6.90 mL of a 1.6 M solution in hexanes,
11.0 mmol) was added dropwise with stirring. The resulting
yellow solution was allowed to react for 40 min while warming to
room temperature, at which time it was cannulated into a
Schlenk flask containing a suspension of CrCl3 (1.60 g, 10.1
mmol) in THF (30 mL) and was left to stir at room temperature
overnight. NaCp (5.50 mL of a 2.0 M solution in THF, 11.0
mmol) was added, and the solution was again left to stir over-
night at room temperature. The solvent was removed in vacuo,
and the residue was extracted with 65 mL of hexanes, 30 mL of
Et2O, and 25 mL of dichloromethane, filtered through Celite,
and rinsed with hexanes (2 ꢀ 10 mL). The resulting green solu-
tion (orange to transmitted) was concentrated and cooled to
-20 °C to yield black crystals of 4d (4.11 g, 79.4%) isolated in
three crops over several days. Anal. Calcd for C30H38N2CrCl: C,
70.09; H, 7.45; N, 5.45. Found: C, 69.82; H, 7.17; N, 5.07. Mp:
195-197 °C. UV/vis (hexanes; λmax, nm (ε, M-1 cm-1)): 420
(8670), 585 (875).
Kinetics Measurements. All kinetics measurements were per-
formed according to the same experimental protocol. A typical
experiment is described as a representative example with 1b. In a
glovebox, 19.1 mg (0.0452 mmol) of 1b was dissolved in pentane
and diluted to the mark in a 100 mL volumetric flask. An aliquot
of 5 mL of this 4.52 ꢀ 10-4 M solution was transferred using a
pipet to a 25 mL volumetric flask. Pentane was added, and then
400 μL of a 2.0 M solution of MeI in MTBE (0.80 mmol) was
added by microliter syringe. Pentane was added up to the mark,
and the solution was thoroughly mixed. A portion of the 9.04 ꢀ
10-5 M solution was loaded into the UV-visible cell for air-
sensitive samples (described above) and transferred from the
glovebox to the spectrophotometer, where the absorption at
530 nm was recorded every 0.5 min for 60 min. The resulting
kinetics trace was fit to a first-order decay curve to give the rate
constant kobs =8.05 ꢀ 10-5 s-1. The pseudo-first-order experi-
ment was repeated three more times with varying excess con-
centrations of MeI. The second-order rate constant, k=(2.4 (
0.3) ꢀ 10-2 M-1 s-1, for the reaction was extracted from the
slope of the straight line plot of kobs vs MeI concentration (R2=
0.9674). The reactions with 1c and 1d were performed in the
same manner, with R2=0.9801 and R2=0.9966, respectively. In
the previously described kinetics measurements for 1a, the
reaction was monitored by the disappearance of the strong peak
of the Cr(II) starting material at 430 nm, rather than the growth
of the Cr(III) methyl product at 530 nm. Since 2 equiv of Cr(II)
are consumed for each equivalent of Cr(III) methyl produced
(Scheme 1 and eq 5), the rate constant previously reported for 1a
was twice as large as those determined in the current study. The
previous value for 1a was divided by 2 in the comparisons with
1b-d, above.
X-ray Crystallography. Data Collection: All crystals were
mounted on a glass fiber and measurements were made
on a Rigaku/ADSC diffractometer for 2b, 2c, and 3b
(Table 1) and on a Bruker X8 APEX diffractometer for 3c,
3d, 4c, and 4d (Table 2) with graphite-monochromated Mo
KR radiation. The data were collected at a temperature of
-100.0 ( 0.1 °C.
Data Reduction: Data for 2b, 2c, and 3b were collected using
the d*TREK61 software package and processed using Twin-
Solve.62 Data for 3c, 3d, 4c, and 4d were collected and integrated
using the Bruker SAINT63 software package. Data were cor-
rected for absorption effects using the multiscan technique
(TwinSolve for 2b, 2c, and 3b and SADABS64 for 3c, 3d, 4c,
and 4d). The data were corrected for Lorentz and polarization
effects.
Structure Solution and Refinement: The structures were
solved by direct methods.65 All non-hydrogen atoms were
refined anisotropically (unless otherwise mentioned below).
All hydrogen atoms were included in calculated positions but
not refined (unless otherwise mentioned below). All refinements
were performed using SHELXL-9766 for 2b, 2c, and 3b and
SHELXTL67 for 3c, 3d, 4c, and 4d. For complexes 2b and 3b, all
Cp hydrogen atoms were located in difference maps and refined
isotropically. Complex 2c crystallizes with a half-molecule on a
mirror plane perpendicular to the b-axis. All four methyl groups
on the xylyl substituents in complex 3b (C7, C8, C20, and C21)
had disordered methyl hydrogens, which were modeled in two
orientations with equal populations. The methyl hydrogens on
C12 and C22 in complex 3c were modeled as disordered in two
orientations with 50% occupancy. In complex 3d, one ethyl
substituent (C25) was disordered and was modeled in two
orientations. The possibility that complex 3d contains a very
small amount (∼1%) of 2d as a cocrystallized impurity50 is
discussed above. Complex 4c 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/SQUEEZE68 program was em-
ployed to search the cell for solvent accessible voids and then to
correct the raw diffraction data to eliminate any residual
electron density found in those voids. The results from this
procedure removed 127 residual electron density from the unit
cell, or approximately 16 electrons per asymmetric unit. This is
less than one molecule of hexane or CH2Cl2 per asymmetric
unit. Since 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; no assumptions were made as to the
identity of the lattice solvent. Complex 4c crystallizes as a
(61) d*TREK, Area Detector Software. Version 4.13; Molecular
Structure Corporation, 1996-1998.
(62) CrystalClear 1.3.6: Mar 19 2004; Rigaku: 1998-2004.
(63) SAINT, Version 7.46A; Bruker AXS Inc.: Madison, WI,
1997-2007.
(64) SADABS. Bruker Nonius area detector scaling and absorption
correction, V2.10; Bruker AXS Inc.: Madison, WI, 2003.
(65) SIR97. Altomare, A.; Burla, M. C.; Camalli, M.; Gascarano,
G. L.; Giacovazzo, C.; Gualiardi, A.; Moliterni, A. G. G.; Polidori, G.;
Spagna, R. J. Appl. Crystallogr. 1999, 32, 115-119.
(66) Sheldrick, G. M. SHELXL-97, Programs for Crystal Structure
€
Analysis (Release 97-2); University of Gottingen: Germany, 1997.
(67) SHELXTL, Version 5.1; Bruker AXS Inc.: Madison, WI, 1997.
(68) SQUEEZE. Sluis, P. v. d.; Spek, A. L. Acta Crystallogr. A 1990,
46, 194-201.

6806 Organometallics, Vol. 28, No. 23, 2009
MacLeod et al.
racemic twin, with both enantiomers present in the crystal. In
complex 4d, one ethyl substituent (C14 and C15) was disordered
and was modeled in two orientations with roughly equivalent
populations. Refinements converged with R1 = 12.4%. The
program ROTAX69 was used to look for possible nonmerohe-
dral twinning. The twin law corresponding to 180 degree twin-
ning about the 0 1 1 reciprocal lattice direction was used to
generate an HKLF5 format data set. Refinements using this
data along with a refined batch scale factor allowed the model to
converge with R1 =5.8%.
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.
Supporting Information Available: Complete crystallographic
data for complexes 2b, 2c, 3b, 3c, 3d, 4c, and 4d. This material is
available free of charge via the Internet at http://pubs.acs.org.
(69) Parsons, S.; Gould, B. ROTAX; University of Edinburgh with
additions by Cooper, R. (Oxford) and Farrugia, L. (Glasgow), Version
Nov 26, 2001.