Cyclopentadienyl mesityl complexes of chromium(ii) and chromium(iii)

Article abstract of DOI:10.1039/c0dt01399d

Chromium(iii) mesityl complexes were synthesized by protonolysis of chromocene with 1,3-diisopropylimidazolium chloride or DBU hydrochloride, salt metathesis with MesMgBr, and single electron oxidation with iodine.

Full text of DOI:10.1039/c0dt01399d

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Cyclopentadienyl mesityl complexes of chromium(II) and chromium(III)†
Wen Zhou,a Jeffrey A. Therrien,a D. Lee K. Wence,a Emily N. Yallits,a Julia L. Conway,a Brian O. Patrickb and
Kevin M. Smith*a
Received 15th October 2010, Accepted 19th October 2010
DOI: 10.1039/c0dt01399d
Chromium(III) mesityl complexes were synthesized by
protonolysis of chromocene with 1,3-diisopropylimidazolium
chloride or DBU hydrochloride, salt metathesis with Mes-
MgBr, and single electron oxidation with iodine.
Cross-coupling catalysts based on first-row transition metals have
demonstrated reactivity that is complementary to established
palladium catalysts.1 The synthesis of well-defined complexes has
provided useful mechanistic details for the cross-coupling reac-
tions of iron,2 cobalt,3 and nickel4 complexes. Organochromium
compounds are known to form Cr(I) species via reductive elimina-
6
tion from Cr(III).5,6 Substituted CpCr(h -arene) complexes such as
A constitute a relatively well-explored class of Cr(I) compounds.7,8
Scheme 1 Reagents and conditions: (i) 1,3-diisopropylimidazolium chlo-
As part of our investigation of mid-valent chromium reagents for
ride, THF, 24 h; (ii) 1.2 MesMgBr, Et2O, 2.5 h; (iii) 0.55 I2, THF : Et2O, 20
C–C bond forming reactions,9 we would like to communicate the
h; (iv) 2 PbCl2, THF, 1 h.
synthesis of Cr(III) mesityl compounds of type B. The synthesis
of the Cr(III) aryl halide complexes without formation of Cr2(m-
NHC)Cl2 (4), as illustrated in Scheme 1. A related complex has
previously been prepared by Jolly and co-workers from 1,3,4,5-
tetramethylimidazol-2-ylidene and CpCrCl2(THF).12
X)2 dimers is achieved via salt metathesis of Cr(II) precursors with
strong neutral L ligands, followed by single electron oxidation.10
(1)
Reaction of chromocene with 1,3-diisopropylimidazolium chlo-
ride in THF (Scheme 1) provides bright purple CpCr(iPr-NHC)Cl
(1) (iPr-NHC = 1,3-diisopropylimidazol-2-ylidene), analogous to
the synthesis by Tilset and co-workers of the bulkier 2,4,6-
Me3C6H2 N-substituted (Mes-NHC) complex.11 The Cr(II) chloro
species 1 reacts smoothly with MesMgBr in Et2O to give orange
CpCr(iPr-NHC)Mes (2). Oxidation of 2 with one-half equiv.
of iodine gives the deep purple Cr(III) iodo complex CpCr(iPr-
NHC)(Mes)I (3). Oxidation with excess PbCl2 provides a di-
rect route from 1 to the Cr(III) dichloro complex CpCr(iPr-
The corresponding Cr(III) diiodo complex (5), bearing an even
less bulky N-heterocyclic carbene ligand, can be prepared by
the reaction of [Cp2Cr]+I- with 1,3-dimethylimidazolium iodide
(eqn (1)). Selective protonolysis of just one cyclopentadienyl ring
is more readily achieved using the cationic Cr(III)[Cp2Cr]+X-
precursor13 than from neutral Cr(II) Cp2Cr.14 Reaction of
[Cp2Cr]+X- with neutral b-diketones, salicylaldimines and b-
ketoimines provides a convenient synthetic route to CpCr(LX)X
complexes without resorting to salt metathesis. The structures
of CpCr[OC(Ph)CHC(Me)O]I (6a), CpCr[OC6H4CHN(2,6-
iPr2C6H3)]I
(6b),
and
CpCr[OC(Ph)CHC(Me)N(3,5-
aDepartment of Chemistry, University of British Columbia Okanagan,
75 3333 University Way, Kelowna, BC, Canada. E-mail: kevin.m.
smith@ubc.ca; Fax: +1 250 807 8005; Tel: +1 250 807 9933
Me2C6H3)](O3SCF3) (6c) have been determined using single-
crystal X-ray crystallography (see ESI†).
bDepartment of Chemistry, University of British Columbia, Vancouver, BC,
Canada
Scheme 2 illustrates the synthesis of analogues of complexes
1, 2 and 3 where the NHC has been replaced with a neu-
tral amidine ligand.15 Reaction of chromocene with DBU·HCl
(DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) in THF generates
purple CpCr(DBU)Cl (7). Isolated 7 reacts with MesMgBr
to afford CpCr(DBU)Mes (8), which can also be synthesized
† Electronic supplementary information (ESI) available: Full experimen-
tal details, and X-ray crystallographic data for complexes 2, 6a, 6b,
6c, 8, 9, and 10. CCDC reference numbers 791384–791390. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01399d
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Scheme 2 Reagents and conditions: (i) DBU·HCl, THF, 20 h; (ii) 1.1
MesMgBr, Et2O, 24 h; (iii) 0.5 I2, THF : Et2O, 4 h.
Fig. 1 Thermal ellipsoid plot (50%) of one of two independent molecules
of 10 without H atoms.
from CrCl2 by sequential addition of NaCp, 5 equiv. DBU
and MesMgBr. Treatment of 8 with one-half equiv. of iodine
is accompanied by a rapid colour change to a deep indigo.
Recrystallization from Et2O at -35 ◦C results in single-crystal
X-ray quality crystals of CpCr(DBU)(Mes)I (9).
the unit cell are remarkably linear compared to the bent geometries
observed in previously structurally characterised [CpCr(m-X)]2
and [Cp*Cr(m-X)]2 complexes.14,19,20
The kinetics and thermodynamics of the interconversion of
h -arene Cr(I) A and Cr(III) mesityl B are expected to vary
The violet CpCr(L)Cl complexes 1 and 7 have UV-vis bands
with lmax of 417 nm and 547 nm, respectively, comparable to the
previously reported value of 535 nm for CpCr(Mes-NHC)Cl.11
Replacing the chloride ligand with an aryl group shifts the band to
higher energy (lmax = 464 nm and 496 nm for 2 and 8, respectively),
again consistent with Tilset’s CpCr(Mes-NHC)Ph compex.11 The
darker blue colour of 4 (lmax = 579) is characteristic of CpCr(L)Cl2
species.11,12,16 The 1H NMR signals of the Cr(II) and Cr(III)
complexes are broad and shifted as expected for paramagnetic
organochromium complexes. The downfield shifted imidazolin-2-
ylidene ring protons11 and the upfield shifted iPr methyl groups of
the iPr-NHC complexes are perhaps the most diagnostically useful
resonances observed.
6
considerably as the L and X ligands are modified. Reductive
elimination from octahedral Cr(III) complexes is typically preceded
by ligand loss.5,6 The thermodynamics of oxidative addition and
reductive elimination is critically dependent on the nature of
i
the aryl–X bond.21 With L = Pr-NHC and X = I, complex
3 is positioned close to one end of a hypothetical continuum
between A and B. We are currently developing synthetic routes
to incorporate more labile neutral L donor and more strongly
electron-donating anionic X ligands to explore the fundamental
chemistry of relevance to cross-coupling reactivity.
We thank NSERC, CFI and UBC Okanagan for financial
support, K. Cory MacLeod for assistance with the synthesis of
8 and 9 and Anita Lam for collection of single-crystal X-ray
crystallographic data for 2.
The single-crystal X-ray crystal structures of 2 and 8 confirm
their monomeric, two-legged piano stool geometries, with the
chromium lying in the plane defined by the Cp centroid, the
mesityl ipso carbon, and the DBU or NHC donor atom. The
Notes and references
˚
˚
Cr–C(ipso) bond lengths of 2.0995(13) A and 2.0931(19) A for 2
and 8, respectively, is similar to related Cr(II) aryl complexes.11,17
The single-crystal X-ray crystal structure of 9 reveals a more
distorted three-legged piano stool geometry with a slightly longer
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˚
Cr–C(ipso) bond length of 2.1188(18) A. The Cr–ligand bonds to
DBU and I in 9 (Cr–N = 2.0901(15) and Cr–I = 2.7545(3)) are also
significantly longer than in 8 (Cr–N = 2.0755(16)) and 6a/6b (ave
Cr–I = 2.6614(6)), respectively, presumably due to the increased
steric repulsion in 9 (see ESI†).
Initial attempts to produce single-crystal X-ray quality crystals
of 8 in Et2O at -35 ◦C without additional DBU afforded black
crystals of [CpCr(m-Mes)]2 (10). The role of DBU in the formation
of dimer 10 remains to be determined.18 Complex 10 is more
directly prepared by sequential addition of NaCp and MesMgBr
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molecules of 10 is shown in Fig. 1. The Cr–Cr distances of
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˚
2.5496(8) and 2.5301(8) A in [CpCr(m–Mes)]2 are longer than those
observed in [CpCr(m-I)]2,19 [Cp*Cr(m-Me)]2,20 or (Cp*Cr)2(m-
Et)(m-Ph),20 but are shorter than those in [Cp*Cr(m-Cl)]2 or
20
[CpCr(m-OCMe3)]2.14 The Cp2Cr2 core of both molecules of 10 in
338 | Dalton Trans., 2011, 40, 337–339
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