Dichloromethane activation by chlorochromium(II) complexes with TpiPr2: Generation of an electrophilic Cr-methylene species without the action of an external Cl-abstraction reagent

Article abstract of DOI:10.1246/cl.2001.1094

Five-coordinated chlorochromium(II) complexes with Tp^iPr2 activate CH₂Cl₂ to give a metal-carbene species without the action of an external Cl-abstraction reagent, and the resulting methylene fragment is trapped by nucleop

Full text of DOI:10.1246/cl.2001.1094

1094
Chemistry Letters 2001
Dichloromethane Activation by Chlorochromium(II) Complexes with TpiPr : Generation of an
Electrophilic Cr–Methylene Species without the Action of an External Cl-Abstraction Reagent
2
Ken-ichiro Sugawara, Shiro Hikichi,* and Munetaka Akita*
Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503
(Received August 6, 2001; CL-010753)
Five-coordinated chlorochromium(II) complexes with TpiPr2
activate CH2Cl2 to give a metal–carbene species without the
action of an external Cl-abstraction reagent, and the resulting
methylene fragment is trapped by nucleophiles such as pyrazole,
pyridine and olefin.
unsaturated chlorochromium(II) complexes to promote methyl-
ene transfer reaction via electrophilic Cr–methylene species
under mild conditions (ambient temperature, without UV-irradia-
tion and using of an external Cl-abstracting reagent).
Coordinatively unsaturated chlorochromium(II) complexes
were prepared by metathesis between KTpiPr and CrCl2 under
2
anaerobic conditions as shown in Scheme 1.7 The mononuclear
II
five-coordinated complexes, TpiPr Cr Cl(L), were formed in the
2
Activation of carbon–halogen bonds by transition metal
compounds is involved in various organic transformation
processes. From the view point of environmental chemistry, acti-
vation of the relatively stable C–Cl bond is also an important sub-
ject related to the degradation of harmful chlorinated compounds.
Coordinatively unsaturated low-valent early-transition metal
species are known to be effective reagents for reductive activa-
tion of haloalkanes due to their low redox potential.1 A family of
hydrotris(pyrazolyl)borates (= TpR) is one of the extensively uti-
lized ligands in inorganic chemistry involving organometallic
field.2 Although a large number of early-transition metal com-
plexes with TpR have been reported, chromium(II) complexes are
still scarce. Recently, Theopold and co-workers reported synthe-
sis and structural characterization of chloro- and
alkylchromium(II) complexes with the highly hindered TptBu,Me
[= hydrotris(3-tert-butyl-5-methyl-1-pyrazolyl)borate] ligand, but
their reactivity except for the oxygenation of the phenyl complex
is unknown.3 During the course of our systematic studies on syn-
thesis and reactivity of the TpRM complexes based on bioinor-
ganic and organometallic viewpoints,4 we have revealed that the
presence of a 2-electron donor (= L) such as 3,5-diisopropylpyra-
zole (= pziPr H; 1a) and pyridine (= py; 1b). The dimeric di(µ-
2
II
chloro) complex, [TpiPr Cr ]2(µ-Cl)2 (1c), containing the virtual-
2
ly square-pyramidal Cr centers bridged by the chloride ligands,
was also obtained successfully in the absence of any additives.8
Interestingly, 1 could activate the C–Cl bonds in CH2Cl2 to
promote the methylene transfer without the use of an external Cl
abstracting reagent. When the mononuclear complexes 1a and
1b were dissolved in CH2Cl2, a mixture of the green
9
dichlorochromium(III) complexes, TpiPr CrCl2(L) (2), and
2
brown species 3 was formed (Scheme 1).10 X-ray crystallograph-
ic analysis11 of the pziPr2H derivative 3a (Figure 1) revealed the
progress of the methylenation of the pziPr H ligand to form the
2
Cr–C–N linkage. Effective magnetic moment of 3a (3.67 µB; a
powder sample, at 298 K) indicated 3a was a high-spin Cr(III)
species (d3, S = 3/2: µspin only = 3.87 µB). The lack of counterions
in the unit cell of 3a implies formation of the zwitterionic structure
of 3a resulting from protonation of the pyrazole fragment.
Moreover, such a zwitterionic character of 3a involving the proto-
nated pyrazolium cation moiety may be stabilized by intramolecu-
lar N–H···Cl hydrogen bonding interaction. In an IR spectrum of
3a, slightly broad νN–H band was observed around 3165 cm–1,
moderately hindered TpiPr [= hydrotris(3,5-diisopropyl-1-pyra-
2
zolyl)borate] ligand can stabilize a coordinatively unsaturated
metal center and, moreover, its somewhat structural flexibility is
expected to allow further substrate activation on the vacant site.
whereas the strong νN–H of the pziPr H ligand of 2a, the
2
N–H(pziPr H) moiety of which is directed to the side opposite to
2
For example, highly coordinatively unsaturated alkyl complexes,
II
TpiPr M R’ (M = Fe, Co, Ni), were carbonylated to give the cor-
the Cl ligands resulting in no hydrogen bonding interaction,
appeared at 3433 cm–1. In addition to the red-shifted νN–H of 3a,
moderately short N42···Cl1 distance (3.086(6) Å) evidenced the
contribution of the intramolecular hydrogen bonding interaction.
2
II
responding acyl–carbonyl complexes, TpiPr M [C(=O)R'](CO)n
2
(n = 1 or 2),5 while the analogous methylcobalt(II) complex hav-
ing the highly hindered TptBu,Me ligand did not show such a reac-
tivity.6 Therefore, combination of chromium(II) and TpiPr is
2
expected to be fruitful for the reductive activation of various sub-
strates. Herein we report the CH2Cl2 activation by coordinatively
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1095
6
7
J. D. Jewson, L. M. Liable-Sands, G. P. A. Yap, A. L. Rheingold, and
K. H. Theopold, Organometallics, 18, 300 (1999).
In contrast to the 2e-donor containing complexes 1a and 1c,
the base-free di-µ-chloro complex 1c indicated the cyclopropana-
tion capability toward external olefin upon dissolution of 1c in
CH2Cl2.12 In general, electrophilic metal–carbene species are
proposed to be a key intermediate for cyclopropanation as well as
formation of zwitterionic complexes like 3.13–15 It should be
noted the present cyclopropanation promoted by 1c is a rare
example via CH2Cl2 activation without the action of an external
Cl abstracting reagent or UV-irradiation.14
IR data (KBr, ν/cm–1): 1a: 3257 (NH), 2532 (BH), 1560 (C=N), 1b:
2554 (BH), 1604 (C=N), 1c: 2545 (BH). The molecular structures of 1
were determined by X-ray crystallography.
8
Theopold and coworkers have reported that the highly hindered
TptBu,Me ligand does not give a dimeric compound like 1c but the
monomeric four-coordinated complex [i.e. (κ3-TptBu,Me)CrIICl],
although the molecular structure of the corresponding pyrazole-contain-
ing complex, TptBu,MeCrIICl(pztBu,MeH), is very similar to that of 1a.
See ref 3a.
Previously reported TpRCrIIICl2(L) with the less hindered TpH (=
2
9
A plausible CH2Cl2 activation mechanism is presented in
Scheme 2. The first C–Cl bond activation yielded the dichloro-
chromium(III) species 2' and the resulting •CH2Cl fragment was
also trapped by 1 to give the chloromethyl intermediate 5. Then,
α-chloride elimination on the coordinatively unsaturated Cr(III)
hydrotris(1-pyrazolyl)borate) and TpMe (= hydrotris(3,5-dimethyl-1-
2
pyrazolyl)borate): TpH2: a) M. J. Abrams, R. Faggiani, and C. J. L.
Lock, Inorg. Chim. Acta, 106, 69 (1985). b) C.-H. Li, J.-D. Chen, L.-S.
Liou, and J.-C. Wang, Inorg. Chim. Acta, 269, 302 (1998). TpMe2: c) T.
Oshiki, K. Mashima, S. Kawamura, K. Tani, and K. Kitaura, Bull.
Chem. Soc. Jpn., 73, 1735 (2000).
center of 5 yielded the carbene intermediate, TpiPr Cr(=CH2)Cl2
10 The pyrazole-adduct of the chlorochromium(II) complex 1a (0.176 g,
0.250 mmol) was dissolved in CH2Cl2 (1 mL). After being stirred for 1
h at room temperature, the solvent was evaporated under vacuum.
Recrystallization from MeCN (–30 °C) afforded the mixture of the
green crystalline solids of 2a (42% yield) and the brown crystals of 3a
(22% yield). Spectroscopic data: 2a: Elemental analysis; Calcd for
C36H62N8BCl2Cr: C, 58.38; H, 8.44; N, 15.13%. Found: C, 57.46;
H, 8.05; N, 15.38%. IR (KBr, ν/cm–1): 2549 (BH), 1576 (C=N). FD-
MS: 739 (M+). µeff (powder, 298 K, µB): 3.96. 3a: Elemental analysis;
Calcd for C37H64N8BCl2Cr: C, 58.88; H, 8.56; N, 14.85%. Found:
C, 58.77; H, 8.29; N, 15.47%. IR (KBr, ν/cm–1): 3165 (NH), 2549
(BH), 1576 (C=N). FD-MS: 753 (M+), 718 (M–Cl+), 166
(CH2pziPr2H+). µeff (powder, 298 K, µB): 3.67. Dissolution of the pyri-
dine derivative 1b in CH2Cl2 resulted in formation of the
dichlorochromium(III) complex 2b (45% isolated yield).
Spectroscopic data for 2b: Elemental analysis; Calcd for
C32H51N7BCl2Cr: C, 57.58; H, 7.70; N, 14.69%. Found: C, 57.98;
H, 7.85; N, 14.12%. IR (KBr, ν/cm–1): 2551 (BH), 1607 (C=N). FD-
MS: 739 (M+). Although we could not isolate the methylenated com-
plex 3b, an FD-MS spectrum of the reaction mixture indicated a peak at
m/z = 94 attributed to CH3–py+ from 3b. Molecular structures of 2a
and 2b were also determined by X-ray crystallography.
11 The complex 3a·(MeCN)3 was crystallized in a orthorhombic space
group Pbca (No. 61) with a = 19.115(1) Å, b = 40.473(3) Å, c =
13.0623(9) Å, V = 10105(1) Å3, Z = 8, Dcalcd = 1.15 g•cm–3. The X-ray
diffraction measurement was made on a Rigaku RAXIS IV imaging
plate area detector with graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) at –60 °C. The structures were solved by the direct method
(SHELXS 86), and refined by full-matrix least squares method (on F2;
SHELXL 97) with anisotropic thermal parameters for all non-hydrogen
atoms. The positions of the hydrogen atoms attached on C1 and N42
were refined. R1 = 0.0879 (for 5001 data with I > 2.0σ(I)) and wR2 =
0.2363 (for 9009 all unique data) with 611 parameters.
2
(4). Finally, the metal–carbene species 4 was trapped by the N-
donating base (i.e. pziPr H and py) to give 3, or the cyclopropana-
2
tion proceeded in the presence of olefin.16
In summary, the coordinatively unsaturated chlorochromi-
um(II) complexes with the moderately hindered TpiPr ligand
2
exhibit activation capability toward CH2Cl2 to promote the meth-
ylene transfer reaction without the action of an external Cl
abstracting reagent. The cyclopropanation using CH2Cl2 as the
methylene source demonstrates the utility of TpiPr to create the
2
coordinatively unsaturated low-valent early-transition metal com-
plexes having the high potential for the reductive activation of
small molecules. Further study on the reactivities of 1 and other
II
TpiPr Cr complexes, which derived from 1, are now under way.
2
12 The base-free complex 1c (52 mg; 0.047 mmol) was dissolved in
CH2Cl2 (2.5 mL) and then 17.3 mmol of styrene was added. After
being stirred for 2.5 h at room temperature, the resulting reaction mix-
ture was analyzed by GC–MS and GC. Yield of cyclopropylbenzene;
2% (based on 1c). Carefully performed blank experiments revealed
that cyclopropanation did not occur in the absence of 1c. Also, such a
methylene transfer from 3 to styrene did not occur.
We are grateful to the Ministry of Education, Culture,
Sports, Science and Technology of the Japanese government for
the financial support of this research (Grant-in-Aid for Scientific
Research for Priority Area: No. 11228201).
13 M. Brookhart and W. B. Studabaker, Chem. Rev., 87, 411 (1987).
14 a) H. B. Friedrich and J. R. Moss, Adv. Organomet. Chem., 33, 235
(1991). b) A. L. Balch, “Homogeneous Catalysis with Metal Phosphine
Complexes,” ed. by L. H. Pignolet, Plenum Press, New York (1983), p
167.
Dedicated to Prof. Hideki Sakurai on the occasion of his
70th birthday.
References and Notes
15 An example of an electrophilic carbene complex of group VI metal with
the TpR ligand, [TpMe2WII(=CH2)(CO)(PhCCMe)]+: a) T. B. Gunnoe, P.
S. White, J. L. Templeton, and L. Casarrubios, J. Am. Chem. Soc., 119,
3171 (1997). b) T. B. Gunnoe, M. Surgan, P. S. White, J. L. Templeton,
and L. Casarrubios, Organometallics, 16, 4865 (1997). c) A. S. Jepsen,
N. J. Vogeley, P. S. White, and J. L. Templeton, J. Organomet. Chem.,
617–618, 520 (2001).
16 An alternate mechanism is direct methylene transfer from the
Cr(III)–carbenoid species 5 to base and olefin via concerted mechanism.
To date, however, no evidence has been obtained for cyclopropanation
through such a direct methylen transfer from transition metal-
chloromethyl (M–CH2Cl) compounds, although the possibility of
nucleophilic substitution of Cl to base on the chloromethyl ligand can-
not be excluded. See ref 14a.
1
2
3
J. K. Kochi, “Organometallic Mechanisms and Catalysis,” Academic
Press, New York (1978).
S. Trofimenko, “Scorpionates – The Coordination Chemistry of
Polypyrazolylborate Ligands,” Imperial College Press, London (1999).
a) J. L. Kersten, R. R. Kucharczyk, G. P. A. Yap, A. L. Rheingold, and
K. H. Theopold, Chem. Eur. J., 3, 1668 (1997). b) A. Hess, M. R. Hörz,
L. M. Liable-Sands, D. C. Lindner, A. L. Rheingold, and K. H.
Theopold, Angew. Chem. Int. Ed., 38, 166 (1999).
Review: a) M. Akita, S. Hikichi, and Y. Moro-oka, J. Synth. Org.
Chem., 57, 619 (1999). b) S. Hikichi, M. Akita, and Y. Moro-oka,
Coord. Chem. Rev., 198, 61 (2000).
4
5
a) N. Shirasawa, M. Akita, S. Hikichi, and Y. Moro-oka, Chem.
Commun., 1999, 417. b) N. Shirasawa, T. T. Nguyet, S. Hikichi, Y.
Moro-oka, and M. Akita, Organometallics, 20, 3582 (2001).