Li[Cp2Zr(C≡CPh)(η2:1,2-PhC2C≡CPh)]: An anionic zirconium(II) intermediate for carbon-carbon coupling

Article abstract of DOI:10.1039/b004478o

The unexpected anionic Zr¹¹ complex [Cp₂Zr(C≡CPh)(η²:1,2-PhC₂C≡CPh)]^- was isolated from LiC≡CPh and Cp₂ZrCl₂ in THF, giving clear evidence for a CC coupling between two alkynyl moieties, one of them being η²-bonded to the zirconium atom.

Full text of DOI:10.1039/b004478o

Li[Cp2Zr(C·CPh)(h2+1,2-PhC2C·CPh)]: an anionic zirconium(II) intermediate
for carbon–carbon coupling
Robert Choukroun,* Jianshe Zhao, Christian Lorber, Patrick Cassoux and Bruno Donnadieu
Equipe Pre´curseurs Mole´culaires et Mate´riaux, Laboratoire de Chimie de Coordination du CNRS, 205 Route de
Narbonne, 31077, Toulouse Cedex, France. E-mail: choukrou@lcc-toulouse.fr
Received (in Basel, Switzerland) 5th June 2000, Accepted 5th July 2000
Published on the Web 24th July 2000
3
The
unexpected
anionic
ZrII
complex
[(C5Me5)2Ti((h -Me3SiC3NC(C·CSiMe3)SiMe3]11b (B). The
tweezer TiIII complex (A) could be described as an intermediate
for the formation of complex (B).
[Cp2Zr(C·CPh)(h2+1,2-PhC2C·CPh)]2 was isolated from
LiC·CPh and Cp2ZrCl2 in THF, giving clear evidence for a
CC coupling between two alkynyl moieties, one of them
being h2-bonded to the zirconium atom.
1
Different crossing reactions were monitored by H NMR to
gain an understanding of the formation of 1.§ When the reaction
of Cp2ZrCl2 and 2 equiv. of LiC·CPh is carried out in THF and
in the absence on sunlight, Cp2Zr(C·CPh)2 2 was obtained as
the sole product and can be kept unchanged at least one week in
the dark, whereas in presence of daylight only 1 is formed.12
Starting from 2 and the solid lithium salt LiC·CPh in THF-d8,
complete consumption of the Li salt gives 1 nearly immediately
and quantitatively in absence or in presence of daylight. We
checked that in the absence of daylight no reaction occurs in
C6D6 between 2 and LiC·CPh (1+1) whereas still in the absence
of daylight the addition of THF-d8 in the NMR tube, which
dissolves the lithium salt, immediately generates 1.
New prospects towards non linear optical (NLO) materials have
induced, in recent years, a resurgence of interest in the
chemistry of acetylenic transition metals complexes.1 Numer-
ous studies have focused on the CC coupling of the acetylenic
moieties, which underlies the synthesis of the building block of
the poly-ynes.2 We have been interested in the reaction of
vanadocene with poly-ynes.3 An unexpected heterodimetallic
complex Cp2V(m+h +h -PhC·CC·CPh))ZrCp2, containing a
butadiene framework with two internal planar tetracoordinate
carbons was isolated from Cp2V and CpA2Zr(C·CPh)2 (CpA =
C5H5, C5H4Me, C5H4But, C5H4SiMe3).4,5 The chemistry of
these bis(alkynyl)metallocene precursors has previously been
detailed6–8 and their synthesis, generally in diethyl ether
solvent, well described.6 On the other hand, a CC bond forming
reaction was reported by Negishi et al.9 as resulting from the
reaction of Cp2ZrCl2 with 3 equiv. of LiC·CPh in THF
followed by hydrolysis affording the isomerically pure (Z)-
1,4-diphenylbut-1-ene-3-yne. In this process, Li[Cp2Zr-
(C·CPh)3] was postulated as an intermediate species.9 The
authors underline the necessity of the third equiv. of LiC·CPh
to Cp2Zr(C·CPh)2 to produce the CC coupling. More recently,
the [Zr(C·CR)3]2 anion moiety was suggested as being an
intermediate species in the trimerization of tert-butyl acetylene
to 1,3,6-tri(tert-butyl)fulvene.10 We report here the X-ray
2
4
Complementary hydrolysis experiments on complex 1 con-
taining the preformed CC coupling were performed to ensure
that the 1,4-diphenylbut-1-en-3-yne is also formed in this case.
In presence of HCl and at room temperature, hydrolysis of 1
leads to the formation of E and Z isomers of the enyne
PhCHNCHC·CPh with a high selectivity when the reaction is
carried out in toluene (toluene: Z+E = 98+2; THF: Z+E =
30+70).¶ Adding LiC·CBut to 2 followed by HCl hydrolysis
gives Z enynes, namely PhCHNCHC·CBut, PhC·CCHNCHBut
and PhCHCHC·CPh (roughly 30, 15, 55% respectively,
1
characterized by GC/MS and H NMR).∑ This result suggests
that the first step of the reaction is the formation of the
structure
of
the
intermediate
anion
species
2
Li[CpZr(C·CPh)(h +1,2-PhC2C·CPh)] and some aspects of its
formation mechanism.
When the reaction of Cp2ZrCl2 with 2 equiv. of LiC·CPh is
carried out in THF, followed by slow diffusion of pentane, the
2
unexpected ZrII lithium salt species Li[Cp2Zr(C·CPh)(h +1,2-
PhC2C·CPh)] 1,† was obtained as a red crystalline complex and
fully characterized by an X-ray structure determination (Fig.
1).‡ The main feature of this structure gives clear evidence for
a CC coupling between two alkynyl moieties, one of them being
Fig. 1 Molecular structure of anionic [1]2 with selected bond distances (Å)
and angles (°), hydrogen atoms omitted. Zr–C(11) 2.206(2), Zr–(C12)
2.342(1), Zr–C(15) 2.314(2), C(11)–C(12) 1.335(2), C(12)–C(13) 1.412(2),
C(13)–C(14) 1.208(2), C(15)–C(16) 1.223(2), Zr–Cp 2.252(av.); Zr–
C(11)–C(111) 147.8(1), Zr–C(15)–C(16) 171.9(1), Zr–C(12)–C(13)
127.9(1), C(12)–C(13)–C(14) 177.5(2), C(11)–Zr–C(12) 33.97(6), C(12)–
Zr–C(15) 88.60(5), C(11)–Zr–C(15) 122.53(5), Zr–C(15)–C(16) 171.9(1),
C(15)–C(16)–C(166) 176.3(2), Cp–Zr–Cp 129.57(av.) [Cp are the centroids
of the C5H5 rings C(1)–C(5), C(6)–C(10)].
2
h -bonded to the zirconium atom. A titanium complex related to
2
1, namely (C5Me5)2Ti(h +1,2-RC2C·CR)] was also recently
isolated by Rosenthal et al. by Mg reduction of (C5Me5)2TiCl2
in the presence of Me3SiC4SiMe3.11a Depending on the ratio of
the reactants, this reaction also affords either the tweezer TiIII
1
complex [(C5Me5)2Ti(h -C·CSiMe3)2][Mg(THF)Cl)] (A) or
DOI: 10.1039/b004478o
Chem. Commun., 2000, 1511–1512
This journal is © The Royal Society of Chemistry 2000
1511

in the same experimental conditions, a red solution is obtained with the
appearance of a paramagnetic ZrIII species (g = 1.997, a(91Zr) = 37 G,
1
20%) which broadens the H NMR signals of the solution (the main peak
observed at 5.6 ppm could not be assigned). Different experiments were
conducted in THF and in absence of daylight (to avoid the formation of the
zirconacyclocumulene species) between (C5H4R)2Zr(C·CPh)2 (R = Me,
SiMe3) and LiC·CPh. 13C NMR spectroscopy shows the characteristic peak
2
of the (h +1,2-PhC2C·CPh) moiety at 208 and 205 ppm for R = Me and
Scheme 1
SiMe3, respectively which suggests the in situ formation of
2
Li[(C5H4R)2Zr(C·CPh)(h +1,2-PhC2C·CPh)] (R
= Me, SiMe3) (for
Cp*2Ti(h +1,2-Me3SiC2C·CSiMe311 two peaks were observed at 227 and
205 ppm). Hydrolysis of the THF mixture with HCl gives the Z/E enynes
(30+70).
2
Li[Cp2Zr(C·CPh)2(C·CBut)] intermediate which could give
2
three possible species such as Li[Cp2Zr(C·CPh)(h +1,2-
PhC2C·CBut)], Li[Cp2Zr(C·CPh)(h +1,2-ButC2C·CPh] and
2
¶ It is of note that Cp2ZrCl2 + 3 equiv. LiC·CPh in toluene at room
temperature for 24 h selectively affords, after hydrolysis, the Z isomer
whereas the same reaction carried out in THF gives a mixture of Z/E isomers
(40+60). The Z isomer was selectively obtained in THF when the reaction
is carried out at 280 °C.9
Li[Cp2Zr(C·CBut)(h +1,2-PhC2C·CPh)], the formation of the
2
latter being favoured by the presence of the less steric R = Ph
2
group on the h chain.**
The CC coupling between two alkynyl moieties from
CpA2Zr(C·CR)2 (CpA = C5H5, R = But; CpA = C5Me5, R = Ph,
SiMe3) has already been demonstrated by Rosenthal
et al.11,13,14 This reaction occurs under hn irradiation or sunlight
4
∑ Hydrolysis experiments with HCl on (C5H4R)2Zr(h +1,2,3,4-
PhCNCNCNCPh) (R = H, Me, SiMe3) give in toluene or in THF solution
nearly 100% of the E isomer PhCHNCH-C·CPh, in contradiction with the
2
described results in which the h coordination is involved.
** LiC·CBut was added to 2 in THF; after stirring for 4 h, the solvent was
evaporated to dryness and replaced by toluene. Hydrolysis with HCl (3
equiv. in solution in diethyl ether) gives Z enynes; 1H NMR (d/ppm, CDCl3,
250 MHz), MS: PhCHNCHC·CBut: 6.55, 5.69, (d, CHNCH, J = 12 Hz),
1.32 (s, But), MS: 184; PhC·CCHNCHBut: 5.87, 5.60, (d, CHNCH, J = 12
Hz), 1.26 (s, But), MS: 184; PhCHNCHC·CPh: 6.70, 5.92, (d, CHNCH, J =
12 Hz), MS: 204. When HCl hydrolysis was performed in THF, a mixture
of Z/E enynes was observed by GC/MS but not further characterized.
†† 1H and 13C{1H} NMR of the reaction show complex spectra in which
three main cyclopentadienyl signals can be observed at 5.76, 5.71,
5.67/105.0, 104.9, 104.7 ppm; low field quaternary carbons at 228, 225.9,
208.2, 205.9, 203, 202 ppm were also observed.
4
to give the zirconacyclocumulene complex CpA2Zr(h +1,2,3,4-
4
RCNCNCNCR).†† Thus (C5H5)2Zr(h +1,2,3,4-PhCNCNCNCPh)
3 should be an excellent candidate for explaining the formation
of 1. Starting from 3, generated by hn daylight in THF-d8 from
2,12 addition of one equiv. of LiC·CPh gives 1. This experiment
is indicative of an equilibrium between the zirconacyclocumu-
2
lene and a (h +1,2-PhC2C·CPh) containing species (Scheme 1)
as already mentioned by Rosenthal et al.13 Nevertheless the
formation of the zirconacumulene species must be catalysed
either by daylight, or by the B(C6F5)3 borane,15 or by the Cp2V
vanadocene for at least one day. By contrast, the formation of
2
the (h +1,2-PhC2C·CPh) moiety in 1 is immediate in THF
1 H. S. Nalwa and S. Miyata, Nonlinear Optics of Organic Molecules and
Polymers, CRC Press, Boca Raton, FL, 1997.
2 (a) A. de Meijeire, Top. Curr. Chem., 1999, 201, 000; (b) F. Diederich,
in Modern Acetylene Chemistry, ed. P. J. Stang and F. Diederich, VCH,
Weinheim, 1995, p. 443; (c) F. Diederich and Y. Rubin, Angew. Chem.,
Int. Ed. Engl., 1992, 31, 1101.
3 (a) R. Choukroun, C. Lorber, B. Donnadieu, B. Henner, R. Frantz and
C. Guerin, Chem. Commun., 1999, 1099 and references therein; (b) R.
Choukroun, B. Donnadieu, I. Malfant, S. Haubrich, R. Frantz, C. Guerin
and B. Henner, Chem. Commun., 1997, 2315.
4 C. Danjoy, J. Zhao, B. Donnadieu, J. P. Legros, L. Valade, R.
Choukroun, A. Zwick and P. Cassoux, Chem. Eur. J., 1998, 4, 1100.
5 R. Choukroun and P. Cassoux, Acc. Chem. Res., 1999, 32, 494.
6 W. Ahlers, B. Temme, G. Erker, R. Fro¨hlich and T. Fox, J. Organomet.
Chem., 1997, 527, 191.
when adding the third LiC·CPh equiv. to the bis(alkynyl)zirco-
nocene complex. No catalytic reaction from 2 to 3 with
LiC·CPh as catalyst was observed by 1H NMR.
At this stage we are not in a position to prove the involvement
of LiC·CPh in a photoassisted reaction with 2 leading to 1.
However, it is noteworthy that when the reaction is carried out
in the dark, it yields only 2. Our results clearly suggest that the
alkynyl coupling reaction is induced by a third alkynyl ligand
via the formation of the unstable ‘ate’ intermediate ZrIV species
Li[Cp2Zr(C·CPh)3], or an assumed tweezer Zr species
[Cp2Zr(C·CPh)2][Li(C·CPh)], which may subsequently re-
arrange to 1.16
7 (a) G. Erker, W. Fro¨mberg, R. Mynott, B. Gabor and C. Kru¨ger, Angew.
Chem., Int. Ed. Engl., 1986, 25, 463; (b) G. Erker, W. Fro¨mberg, R.
Benn, R. Mynott, K. Angermund and C. Kru¨ger, Organometallics,
1989, 8, 911; (c) G. L. Wood, C. B. Knobler and M. F. Hawthorne,
Inorg. Chem., 1989, 28, 382; (d) U. Rosenthal and H. Go¨rls,
J. Organomet Chem., 1992, 439, 36.
8 (a) G. Erker, M. Albrecht, C. Kru¨ger, M. Nolte and S. Werner,
Organometallics, 1993, 12, 4979; (b) A. D. Jenkins, M. F. Lappert and
R. C. Scrivistava, J. Organomet. Chem., 1970, 23, 165.
9 K. Takagi, C. J. Rousset and E. Negishi, J. Am. Chem. Soc., 1991, 113,
1440.
Notes and references
† Spectroscopic data for C42H41LiO2Zr 1: M = 675.9, Calc: C, 74.55; H,
6.06. Found: C, 74.72; H, 5.86%; (40% yield based on 2 equiv. LiC·CPh;
75% yield when the reaction is performed with 3 equiv. LiC·CPh). IR
(Nujol): n(C·C) 2063, 2110 cm21; H NMR (C6D6, d/ppm) 8.26 (pseudo
1
2
triplet, 2H, o-Ph from the h -PhC2-bonded to the zirconium atom), 7.54–6.9
(m, 13H, Ph), 5.78 (s, 10H, Cp), 3.36, 0.95 (m, 16H, THF). A 1H NMR VTP
of the complex from 280 to +80 °C does not show any change in the
solution structure. Assignement of the 13C NMR spectrum of 1 in THF-d8
(d/ppm, J(Hz)) could be tentatively done with a JMOD and 2D
heteronuclear correlation technique (inverse HMQC (LR), gradient se-
10 E. S. Johnson, G. J. Balaich, P. E. Fanwick and I. P. Rothwell, J. Am.
Chem. Soc., 1997, 119, 11 086.
2
lected). Li[Cp2Zr(Ca·CbPh)(h -PhCa·Cb-Cc·CdPh)], 1: 205.9 (s, 3JCH = 4
Hz, Cb), 134.9, 130.4, (s, Ca/Ca), 97.6 (s, Cc), 126.4/107.4 (t, 3JCH = 4–5
Hz, Cb/Cd), 142.5, 127.9, 128.3 (t, 2J = 7–8 Hz, Cipso), 130.8, 129.8, 129.0,
128.4, 128.2, 128.0, 126.1, 126.0, 125.8 (d, 1JCH = 158–162 Hz, Ph), 105.0
(d, 1JCH = 171 Hz, Cp).
11 (a) P. M. Pellny, F. G. Kirchbauer, V. V. Burlakov, W. Baumann, A.
Spannenberg and U. Rosenthal, J. Am. Chem. Soc., 1999, 121, 8313; (b)
P. M. Pellny, F. G. Kirchbauer, V. V. Burlakov, A. Spannenberg, K.
Mach and U. Rosenthal, Chem. Commun., 1999, 2505 and references
therein.
12 R. Choukroun, B. Donnadieu, J. Zho, P. Cassoux, C. Lepetit and B.
Silvi, Organometallics, 2000, 10, 1901.
13 (a) V. V. Burlakov, N. Peulecke, W. Baumann, A. Spannenberg, R.
Kempe and U. Rosenthal, J. Organomet. Chem., 1997, 536, 293; (b) S.
Pulst, P. Arndt, B. Heller, W. Baumann, R. Kempe and U. Rosenthal,
Angew. Chem., Int. Ed. Engl., 1996, 35, 1112.
‡ Crystallographic data for 1: C34H25LiZr·2THF M = 675.95, monoclinic,
space group P21/c, a = 14.986(2), b = 10.4594(8), c = 22.028(2) Å, b =
102.07(1)°, V = 3376(1) Å3, D = 1.33 g cm23, m = 3.61 cm21, R(Rw) =
0.0272 (0.0722) for 4701 unique data and 415 parameters, G.O.F. = 1.04.
Data collection were performed at ca. 160 K on a IPDS STOE
diffractometer using graphite monochromatized Mo-Ka radiation. The
structure was solved by direct methods and subsequent difference Fourier
maps. CCDC 182/1708. See htpp://www.rsc.org/suppdata/cc/b0/b004478o/
for crystallographic files in .cif format.
14 U. Rosenthal, A. Ohff, W. Baumann, R. Kempe, A. Tillack and V. V.
Burlakov, Angew. Chem., Int. Ed. Engl., 1994, 33, 1605.
15 B. Temme, G. Erker, R. Fro¨lich and M. Grehl, Angew. Chem., Int. Ed.
Engl., 1994, 33, 1480.
§ A suspension of Cp2ZrCl2 (0.900 g, 3.08 mmol) was treated with 2 equiv.
solid LiC·CPh (0.665 g, 6.16 mmol) in benzene for 4 h and species such as
1
Cp2ZrCl2, Cp2ZrCl(C·CPh) and 2 were identified by H NMR (in nearly
16 A stable tweezer bimetallic alkynylcopper(I) titanium acetylide has been
1+4+1 ratio respectively). After 24 h stirring and work-up, 2 was obtained
as a crystalline solid (0.840 g, 64% yield). With 3 equiv. LiC·CPh for 24 h,
characterized. M. D. Janssen, M. Herres, L. Zsolnai, D. Grove, A. L.
Spek, H. Lang and G. van Koten, Organometallics, 1995, 14, 1098.
1512
Chem. Commun., 2000, 1511–1512