Table 1 Comparative catalytic data in ethylene oligomerization with AlEtCl2 as cocatalysta
Quantity based on Ni mol
AlEtCl2 (equiv.)
C4
C6
C8
TOF (mol C2H4)/(mol Ni h)
1–butene (%)c
1b
4.07 ¥ 10-5
4.71 ¥ 10-5
4.07 ¥ 10-5
4.00 ¥ 10-5
4.48 ¥ 10-5
4.12 ¥ 10-5
10
10
10
10
10
10
69
68
60
63
78
72
28
28
34
32
20
26
3
4
5
4
2
3
79900
51200
67200
60256
54300
40700
5.8
5.0
8.7
7.4
14.4
8.5
1a
2a
2b
3a
[Et4N]2[NiCl4]a,d
a Ext. temp. = 20 ◦C, C2H4 pressure = 10 bar, solvent = toluene (10 mL), cocatalyst: AlEtCl2 (5 mL toluene solution). [NEt4][PF6] alone was confirmed
to be inactive. Minute amounts of C10 oligomers were typically detected. b In chlorobenzene (10 mL). c Within the C4 fraction. d A negligible activity was
observed in chlorobenzene.
1
Fig. 4 31P{ H} NMR (ppm) spectra of 4 in CD2Cl2.
a sharp peak (SS/RR 65.0 ppm) and a very broad one (RS/SR
ca. 55 ppm, see ESI†), which splits into two doublets at 233 K
2
(one of which is partially masked) with a typical cis JP-P value.
This can be explained as the result of a concerted movement that
exchanges the equatorial and axial positions of the C–C bond
Fig. 5 Diagram of the ion pairing observed in 3·4CHCl3·Et2O. The red
(Fig. 3, bottom right). At 213 K, one of the doublets further splits,
dashed lines represent the shortest H ◊ ◊ ◊ Cl contacts between the cation
consistent with the disorder involving the P2 phosphine (Fig. 3).
and the chlorides. The H1 and H22 atoms were constrained in calculated
In the crystal 3·4CHCl3·Et2O, only the S,S/R,R diastereoisomer is
˚
positions with respect to the parent atoms (C–H distance: 0.98 A).
present and no trace of the R,S/S,R diastereoisomer was detected
in the crude product, at variance with the case of the analogous
dibromide 4. This suggests a remarkable anion effect on the
diastereoselective formation of the bischelate complexes. In 3, the
smaller chloride anion is significantly closer to the metal centre
than the bromide in 4 [Ni ◊ ◊ ◊ Cl distances: 3.623(2) and 4.127(2)
to DBUP.11 Since [Et4N]2[NiCl4] also displays catalytic activity
(Table 1), the equilibria shown in Scheme 2 emphasize the
caution to be exerted when trying to establish structure–properties
relationships.
˚
A; Ni ◊ ◊ ◊ Br distances: 4.102(1) and 4.401(1)], resulting also in
Acknowledgements
closer H1/H22 ◊ ◊ ◊ halogen contacts, which are the shortest found
in 3·4CHCl3·Et2O (see Fig. 5). These non-classical hydrogen bonds
involve the stereogenic CH centre and, although of low energetic
nature, their contribution may be sufficient for the stabilization of
the S,S/R,R diastereoisomer.
We thank Dr Lydia Brelot for the X-ray data collection of
2·3C3H6O, Marc Mermillon-Fournier for technical assistance
and the CNRS and the Ministe`re de la Recherche for funding.
The University of Florida is thanked for its contribution to the
stay of M. O’R. in Strasbourg as part of the France/US REU
programme (Research Experience for Undergraduates).
Our results provide a further example (complex 1) of the
remarkable structural diversity, in solution and in the solid state,
shown by [NiX2(P,N)] (X = halogen, P,N = phosphinoamine)
complexes.9 This has consequences in their reactivity, e.g. in the
catalytic oligomerization of ethylene. Preliminary catalytic results
(Table 1) indicate that 1 and 2 lead to similar activities and
selectivities, suggesting the involvement of a similar active species.
In chlorobenzene, the most soluble isomer 1, present mainly in
the monochelated [NiCl2(DBUP)] form, shows enhanced activity
but the selectivity remains similar to that observed for a toluene
suspension. Complex 3 is also active and gives results similar to
the other catalysts. Complexes 1–3 lead to TOF values similar or
slightly higher than those found for a range of Ni(II) complexes
with P,N chelates10 and ca. 10 times higher than an interesting
Ni(II) complex with a N-phosphinoguanidine P,N ligand related
Notes and references
§ X-Ray diffraction data (173 K, Mo Ka): DBUP: C21H25N2P, M = 336.40,
◦
˚
P21/c, a = 8.6126(5), b = 15.514(1), c = 14.403(1) A, b = 109.505(4) , V =
3
-3
-1
˚
1814.0(2) A , Z = 4, Dc = 1.231 g cm , m = 0.156 mm , F(000) = 720,
Rint = 0.051, R1 (I > 2s) = 0.056, wR2 (I > 2s) = 0.137 (217 param., 2218
obs. refl., 3755 unique); 2·3C3H6O: C51H68Cl4N4Ni2O3P2, M = 1106.25,
˚
P-1, a = 10.6254(3), b = 13.0726(5), c = 19.4149(7) A, a = 82.882(2), b =
◦
3
-3
˚
82.801(2), g = 86.872(2) , V = 2652.79(16) A , Z = 2, Dc = 1.385 g cm ,
m = 1.016 mm-1, F(000) = 1160, Rint = 0.045, R1 (I > 2s) = 0.052, wR2 (I >
2s) = 0.111 (665 param., 8595 obs. refl., 12083 unique); 3·4CHCl3·Et2O:
C50H64Cl14N4NiOP2, M = 1354.00, P-1, a = 14.4443(5), b = 15.8◦472(6),
˚
c = 15.9016(4) A, a = 116.871(2), b = 95.778(2), g = 97.478(2) , V =
3
-3
-1
˚
3166.5(2) A , Z = 2, Dc = 1.420 g cm , m = 0.986 mm , F(000) = 1392,
6094 | Dalton Trans., 2009, 6092–6095
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The Royal Society of Chemistry 2009
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