Synthesis of coordinatively unsaturated cobalt(II)-alkyl complexes bearing phosphorus-bridged [1.1]ferrocenophanes

Article abstract of DOI:10.1246/cl.2006.260

Alkylation of a tetrahedral cobalt(II)-dichloro complex bearing a sterically demanding and electron-donating diphosphine chelate gave a corresponding dialkyl-complex, which is the first coordinatively unsaturated cobalt(II)-(diphosphine)-(dialkyl) complex isolated and characterized by X-ray analysis. This complex is expected to serve as a model intermediate in the cobalt(II)-catalyzed Heck-type reactions. Copyright

Full text of DOI:10.1246/cl.2006.260

260
Chemistry Letters Vol.35, No.3 (2006)
Synthesis of Coordinatively Unsaturated Cobalt(II)–Alkyl Complexes
Bearing Phosphorus-bridged [1.1]Ferrocenophanes
Yuki Imamura,1 Tsutomu Mizuta,ꢀ1 Katsuhiko Miyoshi,ꢀ1 Hideki Yorimitsu,2 and Koichiro Oshima2
1Department of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526
2Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510
(Received December 8, 2005; CL-051511; E-mail: mizuta@sci.hiroshima-u.ac.jp)
Alkylation of a tetrahedral cobalt(II)–dichloro complex
bearing a sterically demanding and electron-donating diphos-
phine chelate gave a corresponding dialkyl-complex, which is
the first coordinatively unsaturated cobalt(II)-(diphosphine)-
(dialkyl) complex isolated and characterized by X-ray analysis.
This complex is expected to serve as a model intermediate in the
cobalt(II)-catalyzed Heck-type reactions.
Coordinatively unsaturated transition-metal-alkyl com-
plexes are important intermediates in stoichiometric and catalyt-
ic reactions applied to a variety of substrate-transformations, and
it is, therefore, essential to reveal their structures and reactivities,
not only to the understanding of these reactions but also to the
development of new relevant reactions. However, these metal–
alkyl complexes except square-planar d8 ones are in general la-
bile, and so only a limited number of them have been character-
ized so far.1 Recent reports concerning cobalt(II)-catalyzed
Heck-type reactions have argued that coordinatively unsaturated
cobalt(II)–dialkyl complexes bearing phosphine ligand(s) play a
vital role as an intermediate in the catalytic cycle.2 However,
such cobalt(II) complexes are too labile to isolate and/or charac-
terize, whereas some of similar cobalt(II)–diaryl complexes have
been found to have a low-spin square-planar structure,3 and sev-
eral iron(II)(diphosphine)(diaryl) complexes have been also pre-
pared.4 We recently established an efficient synthetic route to
novel phosphorus-bridged [1.1]ferrocenophanes in which two
P atoms are linked doubly with two electron-releasing 1,10-ferro-
cenediyl units.5 Since these bulky diphosphines are expected to
exert a peculiar steric demand, an attempt was made in this study
to prepare and characterize some of coordinatively unsaturated
cobalt(II)–alkyl complexes with these double-chained diphos-
phines.
ꢀ
Figure 1. ORTEP drawing of 2a. Selected bond lengths (A)
and angles (ꢁ): Co–Cl 2.2588(7), Co–P1 2.3873(6), Co–P2
2.3742(6), Co–C33 2.013(2); Cl–Co–P1 109.28(3), Cl–Co–P2
108.36(3), Cl–Co–C33 114.43(7), P1–Co–P2 93.15(2), P1–
Co–C33 115.30(8), P2–Co–C33 114.26(7).
SiCH2MgCl devoid of ꢀ-hydrogen atoms in ether, afforded a
mixture of mono- and dialkyl complexes 2a and 3a, respectively,
as precipitates (Scheme 1). Recrystallization from THF/hexane
gave a madder crystal of 2a only. A molecular structure of 2a
was determined by X-ray diffraction6 to have a distorted tetra-
hedral geometry (Figure 1). The P–Co–P bite angle of 93.2ꢁ
was fairly smaller than a tetrahedral angle but was close to that
of 1a, reflecting a steric rigidity of the diphosphine framework.5
ꢀ
The Co–C bond (2.013(2) A) was almost comparable to those
of a related tetrahedral diamine complex [Co(CH2SiMe3)2-
0
0
ꢀ
(tmeda)] (2.025(7) and 2.024(7) A) (tmeda = N,N,N ,N -tetra-
methylethylenediamine).7
Any attempts to obtain a single crystal of the dialkyl com-
plex [CoR2(P-P)] 3a were unsuccessful, but it could be charac-
terized, though paramagnetic, on the basis of 1H NMR spectros-
copy. The 1H NMR spectrum of 3a was quite simple, having on-
ly eight resonance signals.8 Seven of the eight signals appeared
at similar positions to those of the precursor complex 1a,9
indicating that 3a would also adopt a tetrahedral geometry.
The chemical shift of the eighth signal was observed newly at
9.7 ppm (18H, ꢁꢁ1=2 ¼ 100 Hz) due to the CH2SiMe3 groups.10
The protons on the ꢂ-carbon of the CH2SiMe3 groups were not
observed owing to the paramagnetic cobalt center near-by.
In order to manage to establish the molecular structure of the
dialkyl complex, the precursor complex was changed to the
Me3SiCH2-substituted diphosphine complex 1b,11 which is also
rigid but exerts a somewhat different steric demand and possess-
es stronger electron-donating ability than does 1a. Treatment of
1b with Me3SiCH2MgCl in ether yielded a bright yellow-green
complex 3b in 83% yield (Scheme 1), the 1H NMR spectrum of
which showed a new signal assignable to the CoCH2SiMe3
Alkylation of the tetrahedral [CoCl2(P-P)] 1a5c bearing
PPh-bridged [1.1]ferrocenophane with 2 equivalents of Me3-
R
P
Fe Fe
CH2SiMe3
Co
Cl
P
R
P
Fe Fe
R
Me3SiCH2MgCl
ether, 0°C
Cl
Cl
2a: R = Ph
Co
+
P
R
R
P
Fe Fe
1a: R = Ph
1b: R = CH2SiMe3
CH2SiMe3
Co
CH2SiMe3
P
R
3a: R = Ph
3b: R = CH2SiMe3
Scheme 1.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
261
Furthermore, when the reductive elimination takes place, the
P–Co–P bite angle of the diphosphine is concomitantly altered
so as to facilitate it.15 It is, therefore, highly plausible that steric
bulkiness, rigidity and strong electron-donating ability of the
present P-bridged [1.1]ferrocenophanes discourage complexes
3a and 3b from undergoing the reductive elimination. Further
study regarding the reactivities of 3a and 3b is currently under-
way with particular reference to their catalytic activities.
This work was supported by Grant-in-Aid for Scientific
Research (No. 16033245) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We also thank
Prof. K. Inoue, Hiroshima University, for X-ray analysis of 4.
ꢀ
Figure 2. ORTEP drawing of 3b. Selected bond lengths (A)
and angles (ꢁ): Co–P1 2.401(2), Co–P2 2.354(2), Co–C29
2.039(9), Co–C33 2.043(7); P1–Co–P2 92.14(8), P1–Co–C29
112.4(2), P1–Co–C33 117.6(2), P2–Co–C29 117.1(3), P2–Co–
C33 105.9(2), C29–Co–C33 110.5(3).
References and Notes
1
2
3
S. Yoshimitsu, S. Hikichi, M. Akita, Organometallics 2002, 21, 3762,
and references cited therein.
Y. Ikeda, T. Nakamura, H. Yorimitsu, K. Oshima, J. Am. Chem. Soc.
2002, 124, 6514, and references cited therein.
a) J. Chatt, B. L. Shaw, J. Chem. Soc. 1961, 285. b) P. G. Owston,
J. M. Rowe, J. Chem. Soc. 1963, 3411. c) L. Falvello, M. Gerloch,
Acta Crystallogr., Sect. B 1979, B35, 2547.
protons at 8.6 ppm (18H, ꢁꢁ1=2 ¼ 80 Hz),12 together with the
signals due to the diphosphine part.
4
5
E. J. Hawrelak, W. H. Bernskoetter, E. Lobkovsky, G. T. Yee, E. Bill,
P. J. Chirik, Inorg. Chem. 2005, 44, 3103.
a) T. Mizuta, M. Onishi, K. Miyoshi, Organometallics 2000, 19, 5005.
b) T. Mizuta, Y. Imamura, K. Miyoshi, J. Am. Chem. Soc. 2003,
125, 2068. c) T. Mizuta, Y. Imamura, K. Miyoshi, H. Yorimitsu, K.
Oshima, Organometallics 2005, 24, 990. d) Y. Imamura, T. Mizuta,
A single-crystal X-ray analysis demonstrates that 3b is ac-
tually our desired dialkyl complex adopting a tetrahedral geom-
etry (Figure 2) and it is the first coordinatively unsaturated
cobalt(II)(diphosphine)(dialkyl) complex characterized. It is
notable here that the metal center is nicely protected by the
two bulky 1,10-ferrocenediyl units as well as by the methylene
groups. The P–Co–P bite angle of 92.1ꢁ and the Co–P bond
K. Miyoshi, Organometallics 2005, accepted.
.
ꢂ
6
Crystal data for 2a: C36H37ClCoFe2P2Si C4H8O, triclinic, P1, a ¼
ꢀ
10:1590ð2Þ, b ¼ 11:0890ð3Þ, c ¼ 17:3800ð4Þ A, ꢂ ¼ 77:854ð1Þ, ꢀ ¼
ꢀ
lengths of 2.40 and 2.35 A are comparable to those of 1a
75:870ð1Þ, ꢄ ¼ 86:508ð2Þꢁ, V ¼ 1856:08ð8Þ A , Z ¼ 2, Dcalcd
¼
ꢀ 3
ꢁ
5c
ꢀ
(95.6 , 2.39 and 2.37 A) and of 2a (vide supra).
1:499 g cmꢂ1, T ¼ 200 K, R=wR factors (I=ꢅðIÞ > 3:00) 0.0346/
0.0590, CCDC-288272. Crystal data for 3b: C36H60CoFe2P2Si4,
monoclinic, P21=n, a ¼ 10:4850ð4Þ, b ¼ 21:5770ð7Þ, c ¼
For comparison, similar alkylation was conducted on the di-
chlorocobalt(II) complex bearing a much less bulky but much
more flexible dppp ligand (dppp = 1,3-bis(diphenylphosphino)-
propane) used often as a supporting chelate in transition-metal
catalysts. The reaction of [CoCl2(dppp)] 1c with Me3SiCH2-
MgCl in ether afforded a deep-red solution (Scheme 2), which
was allowed to stand overnight to give deep-red crystals 4 in
71% yield (based on dppp).13
ꢁ
ꢀ
ꢀ 3
18:6300ð4Þ A, ꢀ ¼ 96:870ð1Þ , V ¼ 4184:5ð2Þ A , Z ¼ 4, Dcalcd
¼
1:330 g cmꢂ1, T ¼ 200 K, R=wR factors (I=ꢅðIÞ > 3:00) 0.0674/
0.1248, CCDC-288273. Crystal data for 4: C54H52CoP4, monoclinic,
ꢀ
C2=c, a ¼ 18:247ð2Þ, b ¼ 13:138ð1Þ, c ¼ 19:919ð2Þ A, ꢀ ¼
110:203ð2Þ , V ¼ 4481:3ð7Þ A , Z ¼ 4, Dcalcd ¼ 1:310 g cmꢂ1, T ¼
294 K, R=wR factors (I=ꢅðIÞ > 3:00) 0.0344/0.0394, CCDC-288274.
R. S. Hay-Motherwell, G. Wilkinson, B. Hussain, B. Hursthouse,
Polyhedron 1990, 9, 931.
ꢁ
ꢀ 3
7
8
An X-ray analysis revealed 4 to be not an expected co-
balt(II)–dialkyl complex but a [Co0(dppp)2] complex,6 which
is the first homoleptic cobalt(0)–tetraphosphine complex charac-
terized by X-ray analysis. 4 adopts a distorted tetrahedral geom-
etry withꢁa dppp bite angle of 98.6ꢁ and other P–Co–P angles of
The following 1H NMR data are reported with the chemical shift
followed by the integration, the peak width at half-height in Hertz
(ꢁꢁ1=2) and the peak assignment. 1H NMR of 3a (300.4 MHz, C6D6,
TMS): ꢆ ꢂ13:6 (4H, 370 Hz, o-Ph), ꢂ7:25 (4H, 48 Hz, C5H4), ꢂ3:61
(2H, 24 Hz, p-Ph), 5.55 (4H, 30 Hz, C5H4), 6.65 (4H, 30 Hz, m-Ph),
9.7 (18H, 100 Hz, CoCH2SiMe3), 16.2 (4H, 450 Hz, ꢂ-C5H4), 29.69
(4H, 24 Hz, C5H4).
ꢀ
102–125 . The Co–P bonds (2.17 A on average) are much short-
9
1H NMR of 1a (300.4 MHz, CDCl3, TMS): ꢆ ꢂ9:6 (4H, 200 Hz,
o-Ph), ꢂ7:00 (4H, 23 Hz, C5H4), ꢂ2:49 (2H, 15 Hz, p-Ph), 5.72
(4H, 15 Hz, C5H4), 10.34 (4H, 16 Hz, m-Ph), 12.4 (4H, 240 Hz,
ꢂ-C5H4), 26.58 (4H, 10 Hz, C5H4).
er than those of the cobalt(II)–alkyl complexes 3b and 2a and
other usual CoII–P bonds,14 which is attributable to an extensive
ꢃ-back donation from the cobalt(0) center to the P atom(s).
Unexpected formation of 4 is probably brought about by the
reductive elimination (coupling) of the two alkyl groups on the
Co(II) center, followed by the scrambling of the resulting
unstable Co0(dppp) units. In fact, the elimination product Me3-
SiCH2CH2SiMe3 was detected in the reaction mixture along
with some Si-containing by-products by GC-MS analysis.
10 A. R. Hermes, G. S. Girolami, Organometallics 1987, 6, 763.
11 1H NMR of 1b (300.4 MHz, CDCl3, TMS): ꢆ ꢂ8:44 (4H, 26 Hz,
C5H4), ꢂ3:44 (18H, 28 Hz, PCH2SiMe3), 4.76 (4H, 20 Hz, C5H4),
8.1 (4H, 260 Hz, ꢂ-C5H4), 27.38 (4H, 7 Hz, C5H4).
12 1H NMR of 3b (300.4 MHz, C6D6, TMS): ꢆ ꢂ4:93 (4H, 46 Hz, C5H4),
ꢂ2:33 (18H, 17 Hz, PCH2SiMe3), 4.23 (4H, 22 Hz, C5H4), 8.6 (22H,
80 Hz, CoCH2SiMe3 and ꢂ-C5H4), 26.86 (4H, 15 Hz, C5H4), 43.4
(4H, 390 Hz, PCH2SiMe3).
13 1H NMR of 4 (300.4 MHz, C6D6, TMS): ꢆ 2.7 (160 Hz), 4.0 (70 Hz),
7.7 (150 Hz), 9.68 (19 Hz).
14 a) R. L. Carlin, R. D. Chirico, E. Sinn, G. Mennenge, L. Jongh, Inorg.
Chem. 1982, 21, 2218. b) T.-J. Park, S. Huh, Y. Kim, M.-J. Jun, Acta
Crystallogr., Sect. C 1999, C55, 848. c) K. Heinze, G. Huttner,
L. Zsolnai, P. Schober, Inorg. Chem. 1997, 36, 5457.
Ph2
P
Ph2
P
Ph2
P
Me3SiCH2MgCl
Cl
Cl
Co
Co
ether, 0°C
P
P
P
Ph2
Ph2
Ph2
1c
4
15 T. A. Albright, J. K. Burdett, M.-H. Whangbo, in Orbital Interactions
in Chemistry, John Wiley & Sons, New York, 1985, Chap. 19.5.
Scheme 2.
Published on the web (Advance View) January 28, 2006; DOI 10.1246/cl.2006.260