Cyclopentadienyl benzamidinato chromium complexes as models for alkyl halide activation by chromium reagents

Article abstract of DOI:10.1039/b207710h

Cyclopentadienyl complexes of Cr(II) and Cr(III) are stabilized by bis(trimethylsilyl)benzamidinato ligands, allowing the resulting well-defined compounds to serve as models for alkyl halide activation by mid-valent Cr-based reagents.

Full text of DOI:10.1039/b207710h

Cyclopentadienyl benzamidinato chromium complexes as models for
alkyl halide activation by chromium reagents†
Amanda J. Gallant,a Kevin M. Smith*a and Brian O. Patrickb
a Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown, PE,
Canada C1A 4P3. E-mail: kmsmith@upei.ca; Fax: (902) 566-0632; Tel: (902) 566-0375
b Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z1
Received (in Cambridge, UK) 7th August 2002, Accepted 11th October 2002
First published as an Advance Article on the web 4th November 2002
Cyclopentadienyl complexes of Cr(II) and Cr(III) are stabi-
lized by bis(trimethylsilyl)benzamidinato ligands, allowing
the resulting well-defined compounds to serve as models for
alkyl halide activation by mid-valent Cr-based reagents.
While CrCl2 has been used in C–C bond forming reactions for
many years, additional incentive to develop ancillary ligands for
these systems has been provided by the recent discovery of
catalytic1 and asymmetric catalytic2 Cr-based reactions. The
systematic investigation of the relevant Cr(II) and Cr(III) species
is hindered by their paramagnetism, which reduces the utility of
NMR. Recent work in the rational design of related para-
magnetic Cr(III) olefin polymerisation catalysts has relied
extensively on X-ray crystallographic and density functional
theoretical techniques.3 It is the goal of our research program to
apply this approach to synthetic organic applications of Cr-
based reagents, using well-defined compounds to model the
individual reactions of the proposed catalytic cycles. We would
like to communicate our first results in this area, involving the
synthesis, structural determination, computational investigation
and preliminary reactivity studies of cyclopentadienyl benzami-
dinato complexes of Cr(II) and Cr(III) as models for alkyl halide
activation by Cr reagents.
CpCrCl2(THF) has previously been demonstrated to be an
effective precatalyst for coupling organic halides and aliphatic
Fig. 1 Molecular structures of CpCr[(Me3SiN)2CPh]Cl 1a, CpCr[(Me3-
SiN)2CPh](CH2Ph) 2a, CpCr[(Me3SiN)2CC6H4CF3](CH2Ph) 2b, and
aldehydes.1 We chose to modify this structure by incorporating
N-silylated benzamidinato groups, as these readily modified
ligands have previously been shown to stabilize an impressive
array of transition metal and main group complexes.4 As shown
in Scheme 1, the blue Cr(III) chloro complexes CpCr[(Me3-
SiN)2CAr]Cl, (1a, Ar = Ph; 1b, Ar = 4-C6H4CF3) are
synthesized by the reaction of Li[(Me3SiN)2CAr](tmeda)5 with
CpCrCl2(THF).† Both 1a and 1b react with PhCH2MgCl to
give the corresponding purple Cr(III) benzyl complexes,
CpCr[(Me3SiN)2CAr](CH2Ph), (2a, Ar = Ph; 2b, Ar =
4-C6H4CF3). The structures of 1a, 2a, and 2b have been
confirmed by single crystal X-ray diffraction, as shown in Fig.
1.‡
CpCr[(Me3SiN)2CPh] 3a.
products CpCr[(Me3SiN)2CAr], (3a, Ar = Ph; 3b, Ar =
4-C6H4CF3) are highly pentane soluble and are very air
sensitive, which has hindered their isolation and character-
ization. However, crystals of 3a were structurally characterized
by X-ray diffraction (Fig. 1).‡ The formation of 3a and 3b
presumably proceeds via the Cr(III) intermediate CpCr[(Me3-
SiN)2CAr](allyl) that then decomposes to Cr(II) by formal loss
of allyl radical.6 These Cr(II) compounds are apparently stable
with respect to ligand exchange to form Cp2Cr and Cr[(Me3-
SiN)2CAr]2,7 in contrast to the instability of CpCrCl.8
The Cr(III) chloro compounds 1a and 1b are reduced to Cr(II
)
by treatment with allyl magnesium bromide. The red–brown
Density functional theory was employed to investigate this
system, using Gaussian 98 for Windows9 with the B3LYP
Scheme 1 Synthesis of 1, 2 and 3 (a, Ar = Ph; b, Ar = 4-C6H4CF3).
Reagents and conditions: i, [(Me3SiN)2CAr]Li(tmeda), THF; ii,
PhCH2MgCl, THF; iii, C3H5MgBr, THF.
† Electronic supplementary information (ESI) available: experimental and
Fig.
2
Optimised
structures
of
CpCr[(HN)2CH]Cl
(left),
computational details. See http://www.rsc.org/suppdata/cc/b2/b207710h/
CpCr[(HN)2CH](CH2Ph) (middle) and CpCr[(HN)2CH] (right).
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CHEM. COMMUN., 2002, 2914–2915
This journal is © The Royal Society of Chemistry 2002

functional and the LANL2DZ basis set.† The optimised
geometries of the spin quartet CpCr[(HN)2CH]Cl and
CpCr[(HN)2CH](CH2Ph) and spin quintet CpCr[(HN)2CH]
model compounds (Fig. 2) agreed relatively well with the
corresponding structures obtained from X-ray diffraction. The
calculated bond lengths were within 0.035 Å, except for the
cyclopentadienyl Cr–C bonds that were all nearly 0.1 Å too
long. The major variations in R–N–Cr bond angles between the
calculated (R = H) and experimental (R = SiMe3) structures
are most likely due to lack of appropriate steric interactions in
the simplified model system. These parameters improved when
the model was expanded to include SiH3 substituents on the
amidinato nitrogens.†
The nature of the putative allyl intermediate was also
examined computationally. The coordination mode of the allyl
ligand depends on the spin state of the molecule. Starting from
similar initial geometries for CpCr[(HN)2CH](allyl), the spin
quartet optimised with a monodentate C3H5 group while the
allyl ligand in the spin doublet system is bound through all three
carbon atoms (Fig. 3). The high spin quartet configuration was
calculated to be 21.2 kcal mol21 more stable than the doublet.
While hybrid functionals such as B3LYP are known to favour
high spin states due to inclusion of Hartree–Fock exchange,10
reoptimising both geometries using the pure DFT method BP86
still left the doublet species higher in energy than the quartet.
This work was supported by the University of Prince Edward
Island and the Natural Sciences and Engineering Research
Council of Canada (NSERC).
Notes and references
‡ Crystal data for C18H28ClCrN2Si2 1a: M = 416.05, monoclinic, space
group = P21/n, a = 10.1145(3), b = 12.0381(3), c = 18.5467(9) Å, b =
104.055(2)°, V = 2190.6(1) Å3, T = 173 K, Z = 4, m(Mo-Ka) = 7.56
cm21, 19652 reflections measured, 4729 unique, (Rint = 0.039), final
residuals R1 = 0.027 [for 3677 reflections with I > 3s(I)], wR2 = 0.082
[all data]. Crystal data for C25H35CrN2Si2 2a: M = 471.73, monoclinic,
space group = P21/n, a = 11.3146(4), b = 21.3320(7), c = 11.9997(6) Å,
b = 113.493(3)°, V = 2656.2(2) Å3, T = 173 K, Z = 4, m(Mo-Ka) = 5.34
cm21, 22678 reflections measured, 6271 unique, (Rint = 0.049), final
residuals R1 = 0.035 [for 4106 reflections with I > 3s(I)], wR2 = 0.101
[all data]. Crystal data for C26H34CrF3N2Si2 2b: M = 539.73, monoclinic,
space group = C2/c, a = 16.205(2), b = 9.982(2), c = 35.488(3) Å, b =
97.757(4)°, V = 5688(1) Å3, T = 173 K, Z = 8, m(Mo-Ka) = 5.22 cm21
,
17818 reflections measured, 9796 unique, (Rint = 0.038), final residuals R1
= 0.057 [for 4160 reflections with I > 2s(I)], wR2 = 0.141 [all data].
Crystal data for C18H28CrN2Si2 3a: M = 380.60, orthorhombic, space
group = Pbca, a = 9.5040(5), b = 20.006(2), c = 22.638(2) Å, V =
4304.3(5) Å3, T = 298 K, Z = 8, m(Mo-Ka) = 6.44 cm21, 30407
reflections measured, 4284 unique, (Rint = 0.087), final residuals R1 =
0.036 [for 1858 reflections with I > 3s(I)], wR2 = 0.099 [all data]. CCDC
191563–191566. http://www.rsc.org/suppdata/cc/b2/b207710h/ for crys-
tallographic data in CIF or other electronic format.
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Fig. 3 Optimised structures of spin quartet (left) and spin doublet (right)
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The relative energies of reactants and products for benzyl
chloride and allyl chloride activation by the Cr(II) species was
also evaluated, assuming the accepted two step, single electron
transfer mechanism for this process.1,11 Reaction of
CpCr[(HN)2CH] with Cl–R to give R· and CpCr[(HN)2CH]Cl
was calculated to be exothermic in both cases, with DH values
of 24.2 and 27.1 kcal mol21 for R = CH2Ph and C3H5,
respectively. Inclusion of N–SiH3 groups varied the energies
only slightly, to 26.1 kcal mol21 for PhCH2Cl and 29.0 kcal
mol21 for C3H5Cl. Combination of R· and a second equivalent
of CpCr[(HN)2CH] was found to be exothermic, with DH
values of 214.6 and 212.9 kcal mol21 for R = CH2Ph and
C3H5, respectively.
Red–brown solutions of CpCr[(Me3SiN)2CAr] in pentane or
C6D6 react rapidly with PhCH2Cl to give violet solutions. For
3b, the paramagnetic products of this reaction can be observed
using 19F NMR.12 The spectrum consists of two singlets,
assigned to the Cr(III) chloro and benzyl complexes, by
comparison to the 19F NMR spectra of independently syn-
thesised 1b and 2b (262.2 and 262.8 ppm, respectively, C6D6,
referenced to PhCF3 d = 263.72 ppm).13
In conclusion, we have shown that the bis(trimethylsi-
lyl)benzamidinato ligand can be used to prepare a range of well-
defined organometallic chromium complexes. Computational
and preliminary reactivity studies show that these compounds
can act as models for the critical alkyl halide activation step in
Cr-based reagents for organic synthesis. We are currently
investigating how selectively modifying the amidinato sub-
stituents changes the Cr(II)/Cr(III) redox potentials and the
ability of the Cr(II) species to activate organic halides, as well as
other stoichiometric and catalytic reactions of importance to
mid-valent chromium-mediated organic synthesis.
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