C-F bond cleavage and unexpected C-CN activation by cobalt compounds supported with phosphine ligands

Article abstract of DOI:10.1021/om060572d

A cyclometalation reaction involving C-F bond activation at a cobalt(I) center with an aldazine-N atom as an anchoring group, for the first time, affords an ortho-chelated cobalt(III) complex, 1, containing a [C-Co-F] fragment. Under otherwise similar conditions, the first example of an organo cobalt(II) cyanide, 2, formed through Ar-CN bond activation could be obtained using the reaction of 2,6-difluorobenzonitrile with a cobalt(O) complex without C-F bond activation. Both 1 and 2 were characterized through X-ray diffraction.

Full text of DOI:10.1021/om060572d

Organometallics 2006, 25, 4695-4697
4695
C-F Bond Cleavage and Unexpected C-CN Activation by Cobalt
Compounds Supported with Phosphine Ligands
Xiaoyan Li,*,† Hongjian Sun,† Fengli Yu,† Ulrich Flo¨rke,‡ and Hans-Friedrich Klein§
School of Chemistry and Chemical Engineering, Shandong UniVersity, Shanda Nanlu 27, 250100 Jinan,
People’s Republic of China, Department Chemie der UniVersita¨t Paderborn, Warburger Strasse 100,
33098 Paderborn, Germany, and Eduard-Zintl-Institut fu¨r Anorganische und Physikalische Chemie der
Technischen UniVersita¨t Darmstadt, Petersenstrasse 18, 64287 Darmstadt, Germany
ReceiVed June 29, 2006
Summary: A cyclometalation reaction inVolVing C-F bond
actiVation at a cobalt(I) center with an aldazine-N atom as an
anchoring group, for the first time, affords an ortho-chelated
cobalt(III) complex, 1, containing a [C-Co-F] fragment.
Under otherwise similar conditions, the first example of an
organo cobalt(II) cyanide, 2, formed through Ar-CN bond
actiVation could be obtained using the reaction of 2,6-
difluorobenzonitrile with a cobalt(0) complex without C-F bond
actiVation. Both 1 and 2 were characterized through X-ray
diffraction.
Interest in azine and diazine compounds either for pharma-
ceuticals or as building blocks for various applications with
materials and in supramolecular chemistry has led to a variety
of experimental and theoretical studies.5c,8 Cleavage of the N-N
bond in azines by transition metal complexes has been oc-
casionally observed.9,10
Activation of nitriles is a very important issue in organome-
tallic chemistry. Cleavage of C-CN bonds can be promoted
by low-valent nickel complexes.11 Miller reported an efficient
cross-coupling of Grignard reagents with aryl nitrile derivatives
affording the corresponding aryl alkanes or aryl alkenes through
nickel-catalyzed breaking of the C-CN bond.11b Derivatives
of pyridine can be selectively obtained by photochemical
synthesis in cobalt(I)-catalyzed cocyclization of alkynes with
nitriles.12 However, in none of these reports was an Ar-M-
CN intermediate isolated.
Cyclometalation reactions of substrates containing an imine
or a pyridyl anchoring group by iron and cobalt complexes
proceed through intramolecular C-H activation.13
We have recently observed a cyclometalation reaction involv-
ing carbon-fluorine bond activation at a cobalt(I) center with
azine as an anchoring group, which, for the first time, affords
an ortho-chelated cobalt(III) complex containing a C-Co-F
fragment (eq 1).
The activation of carbon-fluorine bonds is of great impor-
tance in organometallic chemistry and catalyst development
because this type of reaction contributes to the fundamental
understanding of the reactivity of stable bonds and the selective
replacement of F atoms.1-4 Reports on C-F activation by first-
row transition metals are rare,5 and no examples of organoiron
or cobalt fluorides containing a C-M-F moiety have been
described. The only cobalt-promoted carbon-fluorine bond
activation was reported with ionic cobaltocenium fluoride as
fluoride source.6 Although C-F activation can be promoted by
Fe2(CO)9 with ortho-halogenated aromatic aldazine ligands, no
intermediate containing a C-Fe-F bonded fragment could be
isolated.7
* To whom correspondence should be addressed. Fax: +86-531-
88564464. E-mail: xli63@sdu.edu.cn.
† Shandong University.
‡ Universita¨t Paderborn.
§ Technische Universita¨t Darmstadt.
(1) (a) Kiplinger, J. L.; Richmond, T. G.; Osterberg, C. E. Chem. ReV.
1994, 94, 373. (b) Saunders, G. C. Angew. Chem., Int. Ed. Engl. 1996, 35,
2615. (c) Burdeniuc, J.; Jedlicka, B.; Crabtree, R. H. Chem. Ber. 1997,
130, 145.
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J.; Fawcett, J.; Holloway, J. H.; Hope, E. G.; Martin, S. M.; Russel, D. R.;
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X.; Font-Bardia, M. Polyhedron 1998, 17, 3927. (e) Lopez, O.; Crespo,
M.; Font-Bardia, M.; Solans, X. Organometallics 1997, 16, 1233. (f) Crespo,
M.; Granell, J.; Font-Bardia, M.; Solans, X. J. Organomet. Chem. 2004,
689, 3088. (g) Su, M.; Chu, A. J. Am. Chem. Soc. 1997, 119, 10178. (h)
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2438. (f) Brezinski, M. M.; Schneider, J.; Radonovich, L. J.; Klabunde, K.
J. Inorg. Chem. 1989, 28, 2414.
Upon combining the ortho-fluorinated aromatic aldazine
ligand N,N′-bis-(2,6-difluorobenzylidene)hydrazone with methyl
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10.1021/om060572d CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/08/2006

4696 Organometallics, Vol. 25, No. 19, 2006
Notes
Figure 1. X-ray crystal structure of 1. Selected bond distances (Å) and angles (deg): Co1-C14 1.907(2), Co1-F4, 1.941(1), Co1-C21
1.993(3), Co1-N2 2.048(2), Co1-P2 2.2082(8), Co1-P1 2.2145(8), N1-C1 1.260(3), N1-N2 1.414(3), N2-C8 1.293(8), C1-C2 1.468-
(3), C8-C9 1.423(3), C14-Co1-F4 176.62(8), C14-Co1-C21 94.3(1), F4-Co1-C21 88.92(9), C14-Co1-N2 82.35(8), F4-Co1-N2
94.41(7), C21-Co1-N2 176.65(9), C14-Co1-P2 91.15(7), C14-Co1-P1 92.19(7), F4-Co1-P2 88.04(5), F4-Co1-P1 88.96(5), C21-
Co1-P2 86.7(1), N2-Co1-P2 92.95(6), N2-Co1-P1 93.29(6), P2-Co1-P1 173.27(3). C21-Co1-P1 87.2(1).
tetrakis(trimethylphosphine)cobalt(I) at room temperature, a
color change from orange to red-brown was observed, indicating
reaction. After workup the mono-ortho-metalated cobalt(III)
fluoride 1 was obtained in 44% yield. Surprisingly, no products
arising from an initial cleavage of the N-N bond7 were found,
at variance with a similar cyclometalation of N,N′-bis(pen-
tafluorobenzylidene)hydrazone with iron observed recently.14
No spontaneous elimination of fluoromethane occurred.
Complex 1 was characterized by elemental analysis, IR and
NMR spectroscopy, and an X-ray diffraction analysis. The
uncoordinated HCN group (ν(CdN) ) 1607.1 cm-1) gives rise
to a singlet at 9.11 ppm, while the coordinated HCN group (ν-
(7)°) are smaller than the corresponding bond angles N1-C1-
C2 (120.3(2)°) and N2-N1-C1 (113.7(2)°) in the uncoordi-
nated moiety. A similar metallocycle of iridium(I) was proposed
as an intermediate in the cyclometalation of a pentafluoroben-
zaldehyde azine phosphorane.15
In view of the many coordination modes observed for a cyano
group, its anchoring properties are hard to predict. To test a
possible C-F bond activation in an ortho-fluorinated benzoni-
trile, pentane solutions of 2,6-difluorobenzonitrile and tetrakis-
(trimethylphosphine)cobalt(0) were combined and kept at room
temperature for 18 h to afford red-brown crystals of 2 in 53%
yield (eq 2).
(CdN) ) 1622.5 cm-1) shows a doublet at 10.75 ppm (4JFH
)
32 Hz). Spectroscopic data are compatible with an octahedral
configuration around the cobalt atom consisting of a P-Co-P
axis and equatorial coordination of C, C, F, and N donor atoms.
The red crystals of 1 under argon decompose at 200 °C and at
ambient temperature in air remain stable for 2 h.
The molecular structure of 1 (Figure 1) confirms the
octahedral geometry around cobalt. The diazine ligand is
coordinated by one of the azine-N atoms and an ortho-chelated
C atom, forming a five-membered metallacycle where the sum
of internal bond angles (540°) indicates planarity. The bite angle
of the chelating ligand [C14-Co-N2 ) 82.35(8)°] is larger
than that found in related compounds.15 The CdN bond length
of the coordinated nitrogen, C8-N2 (1.293(3) Å), is longer than
that of the free CdN, C1dN (1.260(3) Å), while C8-C9 (1.423-
(3) Å) is shorter than C1-C2 (1.468(3) Å), which could be
due to back-donation from the cobalt atom into a ligand π*-
orbital and the considerable conjugation in the cobaltocycle.
The phenyl ring attached to the uncoordinated CdN group is
rotated out of the chelating plane; the dihedral angle formed
between these planes is 43.6(1)°. Within the chelating ring the
bond angles C9-C8-N2 (116.6(2)°) and C8-N2-N1 (111.01-
The CtN stretching vibration is found at 2073 cm-1, which
is close to that of Co(CN)52- species.16 The molecular structure
of 2 shows a distorted square pyramidal coordination of a cobalt-
(II) center with one P atom in apical position. This five-
coordinate geometry is typical for cobalt(II) cyanide complexes,
as are the paramagnetic properties of 2.17 Owing to a steric effect
of the ortho-fluorinated phenyl ring, the angle C1-Co-P3
(108.13(8)°) is larger than 90°, leaving both the P3 atom and
the CtN bond in the same plane with the phenyl ring, which
is perpendicular to the square coordination plane. Although the
strongly electron-withdrawing F-substituents weaken the Ar-
CN bond, no product of the expected C-F activation with Ct
N as an anchoring group was found. To the best of our
(14) Klein, H.-F.; Camadanli, S.; Beck, R. Eur. J. Inorg. Chem.,
submitted.
(15) Perera, S. D.; Shaw, B. L.; Thornton-Pett, M. Inorg. Chim. Acta
1995, 233, 103.
(16) Sweany, R. L. In ComprehensiVe Organometallic Chemistry II; Abel,
E. W., Stone, F. G. A., Wilkinson, G., Eds.; Atwood, J. D., Volume Editor;
Pergamon: New York, 1982-1994; Vol. 8, p 12.
(17) Brown, L. D.; Raymond, K. N. Inorg. Chem. 1975, 14, 1975.

Notes
Organometallics, Vol. 25, No. 19, 2006 4697
period, the reaction mixture turned red-brown in color. After
filtering, the red solid residue was extracted with pentane (80 mL)
and diethyl ether (80 mL), respectively. Repeated recrystallization
from pentane at 4 °C yielded red single crystals suitable for X-ray
structure analysis: 0.62 g (43.5%). Anal. Calcd for C21H29CoF4N2P2
(506.3 g/mol): C, 49.82; H, 5.77; N, 5.53. Found: C, 49.78; H,
5.80; N, 5.57. IR (Nujol): 1622.5, 1607.1 (CdN), 1591.6, 1554.5
1
(CdC), 950.0 (PMe3) cm-1. Dec > 200 °C. H NMR (400 MHz,
d8-THF, 296 K): δ 0.64 (m, 3H, CH3), 0.94 (s, 18H, PCH3), 6.50-
7.44 (m, 6H, Ar-H), 9.11(s, 1H, C1HdN1), 10.75 (d, 4J(HF) ) 33
Hz, 1H, C8HdN2). 13C NMR (100.6 MHz, C6D6, 300 K): δ 10.2
1
(t, | J(PC) + 3J(PC)| ) 12.2 Hz), 105.8 (d, J(CF) ) 19 Hz), 111.3
(d, J(CF) ) 24.2 Hz), 129.9 s, 130.9 s, 134.2 s, 158.9 s, 159.4 s,
159.8 s, 160.3 s, 162.4 s, Ar; 162.9 s, 163.1 s, CHdN. 31P NMR
(162.0 MHz, C6D6, 300 K): δ 8.0 (s).
Synthesis of 2. A solution of 2,6-difluorobenzonitrile (0.65 g,
4.68 mmol) in 10 mL of pentane was combined with a solution of
Co(PMe3)4 (1.68 g, 4.63 mmol) in pentane (50 mL) at -80 °C.
This was allowed to warm to 20 °C and stirred for 18 h to form a
red-brown, turbid mixture, which was filtered. Crystallization from
pentane at 4 °C yielded red-brown single crystals suitable for X-ray
analysis: 1.05 g (53.3%). Anal. Calcd for C16H30CoF2NP3 (426.2
g/mol): C, 64.08; H, 8.34; N, 5.15. Found: C, 64.12; H, 8.30; N,
5.17. IR (Nujol): 2073.5 (CtN), 1586.6, 1549.1 (CdC), 944.5
(PMe3) cm-1. Dec > 134 °C.
Crystallographic Data for 1. C21H29CoF4N2P2, Mr ) 506.3,
crystal dimensions: 0.35 × 0.30 × 0.20 mm, monoclinic, space
group P2(1)/n, a ) 15.3364(6) Å, b ) 9.0040(4) Å, c ) 18.6087-
(7) Å, â ) 109.258(1)°, V ) 2425.86(17) Å3, T ) 298(2) K, Z )
4, Dc ) 1.386 g cm-3, µ ) 0.888 mm-1. Bruker AXS SMART
APEX. A total of 29 413 reflections were collected, 5956 unique
(Rint ) 0.0448), θmax ) 28.20°, multiscan absorption correction.
R1 ) 0.0433 (for 5956 reflections with I > 2σ(I)), wR2 ) 0.1137
(all data). The structure was solved by direct methods and refined
with full-matrix least-squares on all F2 (SHELXL-97) with non-
hydrogen atoms anisotropic.
Figure 2. X-ray crystal structure of 2. Selected bond distances
(Å) and angles (deg): Co1-C7 1.922(3), Co1-C1,1.997(2), Co1-
P1 2.2187(9), Co1-P2 2.2177(9), Co1-P3 2.3075(9), C7-N1
1.141(3), F1-C2 1.375(4), F2-C6 1.386(3); C7-Co1-C1 154.6-
(1), C7-Co1-P2 86.23(8), C1-Co1-P2 89.80(7), C7-Co1-P1
87.56(8), C1-Co1-P1 88.80(7), P2-Co1-P1 162.48(3), C7-
Co1-P3 97.30(8), C1-Co1-P3 108.13(8), P2-Co1-P3 98.49-
(3), P1-Co1-P3 98.54(3).
knowledge, compound 2 is the first example of an organo cobalt-
(II) cyanide formed through Ar-CN bond activation. This type
of metal complex has been synthesized through reaction of
M(PEt3)4 (M ) Ni, Pd, and Pt) with monofluorinated benzoni-
triles.18 Organo nickel cyanides could be possible intermediates
for cyanation19 of aryl bromides and cross-coupling reaction of
aryl halides.11b That the bond activation of Ar-CN suppresses
C-F cleavage in fluorinated nitriles is verified by our work
for cobalt.
Crystallographic Data for 2. C16H30CoF2NP3, Mr ) 426.2,
crystal dimensions: 0.20 × 0.16 × 0.14 mm, monoclinic, space
group P2(1)/n, a ) 12.900(3) Å, b ) 11.552(3) Å, c ) 14.968(3)
Å, â ) 100.001(4)°, V ) 2196.5(8) Å3, T ) 294(2) K, Z ) 4, Dc
) 1.289 g cm-3, µ ) 1.013 mm-1. A total of 12 118 reflections
were collected, 4477 unique (Rint ) 0.0376), θmax ) 26.39°,
semiempirical absorption correction. R1 ) 0.0345 (for 4477
reflections with I > 2σ(I)), wR2 ) 0.0873 (all data). The structure
was solved by direct methods and refined with full-matrix least-
squares on all F2 (SHELXL-97) with non-hydrogen atoms aniso-
tropic. CCDC-600895 and CCDC-600787 (1 and 2) contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB21EZ, UK (fax: (+44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
Experimental Section
General Procedures and Materials. All air-sensitive and
volatile materials were handled either in vacuo or under argon by
using standard Schlenk techniques. Melting points/decomposition
temperatures: sealed capillaries, uncorrected values. N,N′-Bis(2,6-
difluorobenzylidene)hydrazine,14 tetrakis(trimethylphosphine)cobalt-
(0), and methyltetrakis(trimethylphosphine)cobalt(I)20 were prepared
by published procedures. Other chemicals were used as purchased.
All solvents were dehydrated and degassed before use. IR: Nujol
mulls between KBr disks, Bruker spectrophotometer type VECTOR
22. H, 13C, and 31P NMR spectra (400, 100.6, and 162.0 MHz,
respectively) were recorded with a Bruker AVANCE-400 spec-
trometer. For C, H, N analyses an automatic Elementar Vario ELIII
analyzer was used.
1
Acknowledgment. We gratefully acknowledge support by
NSFC, Nos. 20572062 and 20372042, and the doctoral program
of MOE, Nos. 20050422010 and 20050422011.
Synthesis of 1. A solution of 1.07 g (2.83 mmol) of CoMe-
(PMe3)4 in 60 mL of pentane was combined with a solution of
N,N′-bis(2,6-difluorobenzylidene)hydrazine 0.79 g (2.82 mmol) in
pentane (20 mL) at -80 °C. The reaction mixture was allowed to
warm to ambient temperature and stirred for 18 h. During this
Supporting Information Available: Tables containing full
X-ray crystallographic data for 1 and 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
(18) Parshall, G. W. J. Am. Chem. Soc. 1974, 96, 2360.
(19) Schareina, T.; Zapf, A.; Beller, M. Tetrahedron Lett. 2005, 46, 2585.
(20) Klein, H.-F.; Karsch, H. H. Chem. Ber. 1975, 108, 944.
OM060572D