Anion effect in the diastereoselective formation of bischelated Ni(II) complexes with a novel, chiral phosphine derived from 1,8-diazabicyclo[5.4.0] undec-7-ene (DBU)

Article abstract of DOI:10.1039/b909329j

With the new chiral phosphine ligand DBUP, obtained from DBU and Ph ₂PCl, a remarkable anion effect was observed on the diastereoselective formation of the bischelated complexes [Ni(DBUP) ₂](X)_n (X = Cl, Br, n = 2; X = NiC

Full text of DOI:10.1039/b909329j

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Anion effect in the diastereoselective formation of bischelated Ni(II)
complexes with a novel, chiral phosphine derived from
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)†
Matthew O’Reilly,‡ Roberto Pattacini* and Pierre Braunstein*
Received 23rd March 2009, Accepted 27th May 2009
First published as an Advance Article on the web 12th June 2009
DOI: 10.1039/b909329j
With the new chiral phosphine ligand DBUP, obtained from
DBU and Ph2PCl, a remarkable anion effect was observed on
the diastereoselective formation of the bischelated complexes
[Ni(DBUP)2](X)n (X = Cl, Br, n = 2; X = NiCl4, n = 1), which
are active ethylene oligomerization catalyst precursors.
phosphorous species have been reported,4 but their coordination
chemistry has remained unexplored.5
The reaction of Ph2PCl with 2 equiv. DBU in toluene yielded
=
Ph2PCH(CH2)4N(CH2)3N C (DBUP, Scheme 1, Fig. 1) by re-
gioselective phosphorylation at the methylene bonded to the NCN
carbon. A yellow paste precipitated immediately when Ph2PCl was
added to the DBU solution and was unambiguously characterized
as a mixture of [Ph2PN(◊ ◊ ◊C)(CH2)5N(CH2)2(CH2)]Cl (interme-
diate A, Scheme 1, see ESI†) and toluene. In contact with the
DBU present in solution, A was slowly consumed and colourless
DBU·HCl appeared. Consistently, isolated A is rapidly deproto-
nated by DBU to give DBUP, via the aminophosphine B which
was observed by 31P NMR. Intra- or inter-molecular phosphoryl
migration from N to C occurs, as proposed for a phosphonate
derivative.4d When the pale yellow A/toluene paste was left for 24 h
under reduced pressure, [Ph2PCH(CH2)4N(CH2)3NH(◊ ◊ ◊C)]Cl
(DBUP·HCl, Scheme 1) was unexpectedly formed. It is readily
converted into DBUP by treatment with additional DBU. The
A→DBUP·HCl conversion does not take place in homogeneous
The chemistry and catalytic applications of aminophosphines have
generated much attention.1 In the reaction of 2-methyl-2-oxazoline
with Ph2PCl, in the presence of NEt3 as HCl scavenger,2 the sub-
strate was unexpectedly phosphorylated both on the nitrogen and
the exocyclic carbon atoms. We considered that in order to increase
the kinetics of the reaction, it would be advantageous to replace
NEt3 by the more basic DBU (1,8-diazabicyclo[5.4.0]undec-7-ene).
But unexpectedly, phosphorylation was observed on DBU itself,
an unprecedented and particularly surprising feature since DBU is
a well-known and much used organic base. In this reaction, DBU
behaves as a nucleophile, despite its common definition as “non-
nucleophilic” base.3 A P–N bond is initially formed and a 1,3-shift
of the P atom follows to form the P–C bond of the resulting
new alkyl phosphine (Scheme 1), which was isolated and used
as a P,N chelate with Ni(II). Only a few reactions of DBU with
Scheme 1
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS) Universite´ de Strasbourg, 4 rue Blaise Pascal, 67070, Strasbourg
Cedex, France. E-mail: braunstein@chimie.u-strasbg.fr; Fax: +33 3-9024-
132
† Electronic supplementary information (ESI) available: ORTEP plots and
experimental details. CCDC reference numbers 724933 (DBUP), 724930
(2·3C3H6O), 724931 (3·4CHCl3·Et2O) and 724932 (4·6H2O) . For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b909329j
Fig. 1 ORTEP plot of the crystal structure of DBUP. Ellipsoids include
◦
˚
50% of the electron density. Selected distances (A) and angles ( ): P1–C1
1.865(3), C1–C6 1.518(4), N1–C6 1.288(4), N2–C6 1.372(4); N1–C6–N2
125.9(3), N1–C6–C1 117.0(3), N2–C6–C1 117.1(3), C6–N1–C7 115.7(3),
C6–N2–C5 123.1(3), C6–N2–C9 119.8(2), C5–N2–C9 114.9(2).
‡ On leave from the University of Florida, Gainesville, FL, 32611.
6092 | Dalton Trans., 2009, 6092–6095
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
solutions but seems to require the very high concentration existing
in the aforementioned paste.
At variance with the 2-methyl-2-thiazoline2 and 2-amino-2-
thiazoline6 cases, no diphosphine ligand was observed. The
X-ray structure of DBUP (Fig. 1) confirms that the phosphorus
is bonded to the new stereogenic centre C1. The C–N distances
suggest that the double bond is localized between C6 and N1. The
phosphorus lone pair in DBUP is directed anti with respect to
the C1 hydrogen [C16–P1–C1–C6 torsion angle: 171.0(1)◦]. The
N2 atom is slightly pyramidal, the sum of the C–N2–C angles
◦
˚
being 357.8(7) . The P1–C1 bond distance [1.865(3) A] is typical
of alkyl-phosphines.
The reaction of DBUP with [NiCl2(DME)] (DME
=
dimethoxyethane) in CH2Cl2 afforded a red paramagnetic so-
lution. Its rapid evaporation yielded a red, amorphous solid
most likely to be the tetrahedral complex [NiCl2(DBUP)] (1),
but attempts to crystallize it failed. Instead, green crystals of
[Ni(DBUP)2](NiCl4) (2), a formula isomer of 1, were obtained
from MeOH, acetone (Fig. 2) or CH2Cl2. The metal centre in the
dication of 2·3C3H6O is P,N-chelated by two mutually cis DBUP
ligands. It adopts a tetrahedrally distorted square planar geometry
[angle between the P1,Ni1,N1 and P2,Ni1,N3 planes: 15.14(7)◦].
Fig. 3 Diagrams of the crystal structure of 4·6H2O. The red and green
C–C bonds depict the two figures of the disorder (see text). Only the ipso
phenyl atoms are shown. Solvent and hydrogen atoms omitted for clarity.
A complete ORTEP plot is provided in the ESI.†
2-
A NiCl4 group acts as a counterion.
Scheme 2
never been described, to the best of our knowledge.7 In EtOH,
MeOH or water, 2 forms yellow solutions, as the result of NiCl4
Fig. 2 ORTEP plot of the crystal structure of [Ni(DBUP)2]2+ in
2·3C3H6O. Only one of the two disorder images, representing the S,R
diastereoisomer, is depicted. Hydrogen atoms and solvent molecules
omitted for clarity. The NiCl4 dianion (omitted) is located far from the
cationic metal centre. Ellipsoids◦include 50% of the electron density.
2-
solvation,8 and the metal remains bischelated, as confirmed by
comparison of its NMR and IR data with those of [Ni(DBUP)2]Cl2
(3). The latter was independently prepared by a 2:1 reaction
of DBUP with [NiCl2(DME)]. Single crystals of 3·4CHCl3·Et2O
(see below) and of the related dibromide [Ni(DBUP)2]Br2 (4,
in 4·6H2O) were obtained (Fig. 3). The structure of the cation
[Ni(DBUP)2]2+ in 4·6H2O is similar to that in 2. A disorder affects
the ligands (green and red images in Fig. 3) which display two
opposite configurations for the carbon bonded to P1. The two
possible diastereoisomers thus co-crystallized in a 0.7:0.3 ratio
(S,S/R,S and their enantiomers, respectively). A second disorder
involves the seven-membered cycle of phosphine P2 but it does
not affect the configuration of the CH carbon. In contrast to
the situation in the S,S/R,R-diastereoisomer, which displays a C2
pseudosymmetry (Fig. 3, bottom left), the phosphines are not
equivalent in the R,S/S,R diastereoisomer.
˚
Selected distances (A) and angles ( ): Ni1–P1 2.1337(8), Ni1–P2 2.1454(9),
Ni1–N1 1.953(3), Ni1–N3 1.938(2), N1–Ni1–P1 80.41(7), N3–Ni1–P2
83.11(8).
The two possible diastereoisomers of 2 co-crystallized in a 1:1
ratio and are present as two resolved disordered images in the
main residue, as also observed for 4·6H2O (see Fig. 3 and below
for details). Dissolution of the green crystals of 2 in chlorinated
solvents or MeCN reforms a red solution of 1,§ as confirmed by
IR spectroscopy. Crystallization of 2 from solutions containing
1 is quantitative and can be explained by the Le Chatelier’s
principle, implying an equilibrium between the two species in
solution (Scheme 2). Although bischelated Ni(II) complexes
featuring NiCl42- as counterion have been reported in the literature,
equilibria with the parent monochelated NiCl2L2 complex have
1
The 31P{ H} NMR spectrum of 4 (Fig. 4) is consistent with
what is observed in the solid state. At 298 K, the spectrum shows
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6092–6095 | 6093
©

View Article Online
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
This journal is
The Royal Society of Chemistry 2009
©

View Article Online
Rint = 0.050, R1 (I > 2s) = 0.058, wR2 (I > 2s) = 0.148 (678 param.,
7846 obs. refl., 13080 unique); 4·6H2O: C42H62Br2N4NiO6P2, M = 999.43,
5 R. Appel and L. Krieger, J. Organomet. Chem., 1988, 354, 309.
6 G. Margraf, R. Pattacini, A. Messaoudi and P. Braunstein, Chem.
Commun., 2006, 3098.
◦
˚
P21/c, a = 11.9492(3), b = 15.0496(5), c = 25.3293(5) A, b = 90.227(2) ,
3
-3
-1
˚
7 (a) M. E. Cucciolito, V. De Felice, I. Orabona, A. Panunzi and F. Ruffo,
Inorg. Chim. Acta, 2003, 343, 209; (b) G. Exarchos, S. D. Robinson and
J. W. Steed, Polyhedron, 2001, 20, 2951; (c) J. T. Mague and S. W.
Hawbaker, J. Chem. Cryst., 1997, 27, 603; (d) T. Suzuki, A. Morikawa
and K. Kashiwabara, Bull. Chem. Soc. Jpn., 1996, 69, 2539; (e) J. G. H.
Du Preez, B. J. A. M. Van Brecht and I. Warden, Inorg. Chim. Acta,
1987, 131, 259; (f) M. Schaefer and E. Uhlig, Z. Anorg. Allg. Chem.,
1974, 407, 23.
8 N. S. Gill and R. S. Nyholm, J. Chem. Soc., 1959, 3997.
9 For recent examples see, e.g. (a) A. Kermagoret, R. Pattacini, P. Chavez
Vasquez, G. Rogez, R. Welter and P. Braunstein, Angew. Chem. Int.
Ed., 2007, 46, 6438; (b) P. Chavez, I. G. Rios, A. Kermagoret, R.
Pattacini, A. Meli, C. Bianchini, G. Giambastiani and P. Braunstein,
Organometallics, 2009, 28, 1776.
V = 4554.9(2) A , Z = 4, Dc = 1.457 g cm , m = 2.299 mm , F(000) =
2072, Rint = 0.053, R1 (I > 2s) = 0.064, wR2 (I > 2s) = 0.157 (538 param.,
6827 obs. refl., 10110 unique).
1 See e.g. (a) D. Benito-Garagorri and K. Kirchner, Acc. Chem. Res.,
2008, 41, 201; (b) S. J. Roseblade and A. Pfaltz, Acc. Chem. Res., 2007,
40, 1402; (c) P. Braunstein, Chem. Rev., 2006, 106, 134; (d) P. Braunstein,
J. Organomet. Chem., 2004, 689, 3953.
2 R. Pattacini, G. Margraf, A. Messaoudi, N. Oberbeckmann-Winter
and P. Braunstein, Inorg. Chem., 2008, 47, 9886.
3 See e.g. D. Basavaiah, A. J. Rao and T. Satyanarayana, Chem. Rev.,
2003, 103, 811.
4 For recent examples see (a) K. V. P. Pavan Kumar, K. Praveen Kumar,
M. Vijjulatha and K. C. Kumara Swamy, J. Chem. Sci., 2004, 116,
311–317; (b) M. Regitz and U. Bergstraesser, Sci. Synth., 2002, 9, 135–
181; (c) M. Blattner, M. Nieger, A. Ruban, W. W. Schoeller and E.
Niecke, Angew. Chem. Int. Ed., 2000, 39, 2768; (d) A. Kers, I. Kers and
J. Stawinski, J. Chem. Soc. Perkin Trans., 1999, 2071.
10 F. Speiser, P. Braunstein and L. Saussine, Acc. Chem. Res, 2005, 38,
784.
11 P. W. Dyer, J. Fawcett and M. J. Hanton, Organometallics, 2008, 27,
5082.
This journal is
The Royal Society of Chemistry 2009
Dalton Trans., 2009, 6092–6095 | 6095
©