Contrasting bonding modes of a tridentate bis(oxazoline)phosphine ligand in cobalt and iron vs. palladium complexes: Unprecedented N,N-coordination for a N,P,N ligand

Article abstract of DOI:10.1039/b717021c

Unexpected N,N-coordination of the potentially tridentate N,P,N-ligand bis(2-oxazolin-2,5,5-trimethyl)phenylphosphine occurs in Co(ii) and Fe(ii) complexes, in contrast to the P,N- or N,P,N-coordination modes observed in Pd(ii) complexes; this leads to the formation of unprecedented eight-membered ring chelates. The Royal Society of Chemistry 2008.

Full text of DOI:10.1039/b717021c

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Contrasting bonding modes of a tridentate bis(oxazoline)phosphine ligand in
cobalt and iron vs. palladium complexes: unprecedented N,N-coordination for
a N,P,N ligand†‡§
Anthony Kermagoret and Pierre Braunstein*
Received 31st August 2007, Accepted 5th November 2007
First published as an Advance Article on the web 16th November 2007
DOI: 10.1039/b717021c
Unexpected N,N-coordination of the potentially tridentate
N,P,N-ligand bis(2-oxazolin-2,5,5-trimethyl)phenylphos-
phine occurs in Co(II) and Fe(II) complexes, in contrast to
the P,N- or N,P,N-coordination modes observed in Pd(II)
complexes; this leads to the formation of unprecedented
eight-membered ring chelates.
complexes also confirmed the role played by the counter anion on
the coordination behaviour of the N,P,N-ligand bis(2-oxazolin-2-
ylmethyl)phenylphosphine.10
The current interest in cobalt complexes with tridentate ligands
for olefin oligomerization and polymerization11–15 has led us to
use the ligands NOPONMe2 and bis(2-oxazolin-2,5,5-trimethyl)
phenylphosphine (NPNMe2) for the synthesize of new cobalt
complexes (Scheme 1). The reactions of CoCl2 with NOPONMe2
or NPNMe2 in a 1 : 1 molar ratio afforded complexes 1 and
2, respectively.¶ Their magnetic moments of 3.8 and 4.0 lB,
respectively, were determined by the Evans method16,17 in CD2Cl2
and are slightly lower than expected for high spin d7 Co(II)
ions. As observed in the IR spectra of the corresponding Pd(II)
Complexes containing oxazoline heterocycles are much used in
asymmetric catalysis1–5 and P,N-chelating phosphino-oxazoline
ligands form metal complexes, with e.g. Ru(II), Ni(II) and Pd(II),
which are efficient precatalysts in a number of reactions.1,6,7
A
pentacoordinated Ni(II) complex containing a tridentate N,P,N-
ligand of the type bis(oxazolinyl)phenylphosphonite, NOPONMe2
(Scheme 1), catalyses ethylene oligomerisation.8
complexes,9 1 presents two bands for the mC= vibrations at
N
1682 and 1627 cm−1 which correspond to the uncoordinated
and coordinated oxazolines, respectively. The crystal structure
of 1 in 1·0.5C7H8 (Fig. 1) was determined by X-ray diffractionꢀ
and establishes the non-coordination of one oxazoline arm.
The metal centre adopts a distorted tetrahedral coordination
geometry in contrast to the square-planar geometry observed
for Pd(II) complexes with this ligand, either when it behaves
as a P,N-chelate or a N,P,N-tridentate ligand.9 The Ni(II) and
Ru(II) complexes with this ligand, [NiCl2(NOPONMe2)]8 and
[RuCl2(NOPONMe2)]18 are pentacoordinated, with a coordination
Scheme 1 N,P,N ligands and their use for the synthesis of the Co(II)
complexes.
Square-planar palladium(II) complexes coordinated by this
ligand display either a fluxional chelating P,N-coordination mode
1
with chlorides or chloride and alkyl or g -allyl groups as co-
−
ligands or a tridentate N,P,N-coordination mode when BF4
is a counter anion.9 A related study on cationic Ru(II) benzene
Laboratoire de chimie de coordination, Institut de Chimie (UMR 7177
CNRS), Universite´ Louis Pasteur, 4 rue Blaise Pascal, F-67070, Strasbourg,
France. E-mail: braunstein@chimie.u-strasbg.fr; Fax: +33 390 241 322
† The HTML version of this article has been enhanced with colour images.
‡ CCDC reference numbers 659280–659282. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b717021c
Fig. 1 ORTEP view of the molecular structure of 1 in 1·0.5C7H8.
Ellipsoids enclose 50% of the electron density. Only the ipso carbon C1
of the phenyl group attached to P is shown for clarity. Hydrogen atoms
◦
˚
omitted for clarity. Selected bond distances [A] and angles [ ]: Co–N1
2.038(3), Co–P 2.358(1), Co–Cl1 2.238(1), Co–Cl2 2.240(1); N1–Co–P
92.93(9), N1–Co–Cl1 109.2(1), N1–Co–Cl2 108.4(1), P–Co–Cl1 113.44(4),
P–Co–Cl2 112.95(4), Cl1–Co–Cl2 117.06(4).
§ Electronic supplementary information (ESI) available: Detailed experi-
mental procedures for all new compounds. See DOI: 10.1039/b717021c
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geometry intermediate between trigonal-bipyramidal and square-
base pyramidal. This illustrates the bonding versatility of this
˚
˚
ligand. The Co–Cl1 (2.238(1) A), Co–Cl2 (2.240(1) A), Co–P
˚
2.358(1) and Co–N1 (2.038(3) A) distances are similar to those
in Co(II) complexes with a distorted tetrahedral geometry and
containing phosphinito-oxazoline ligands.19
In contrast to the P,N-chelating behaviour of the NOPONMe2
ligand in 1, the crystal structure of 2 in 2·0.5C7H8 (Fig. 2) revealed
ligand chelation by coordination of the two nitrogen atoms of
the tridentate ligand, without the expected coordination of the
phosphorus atom.
Fig. 3 ORTEP view of the molecular structure of 3. Ellipsoids enclose
50% of the electron density. Hydrogen atoms omitted for clarity. Selected
◦
˚
bond distances [A] and angles [ ]: Fe–Cl1 2.2607(8), Fe–Cl2 2.2586(8),
Fe–N1 2.088(2), Fe–N2 2.094(2); N1–Fe–N2 105.80(9), N1–Fe–Cl1
104.79(8), N1–Fe–Cl2 113.21(7), N2–Fe–Cl1 110.91(7), N2–Fe–Cl2
104.39(7), Cl1–Fe–Cl2 117.29(3).
To the best of our knowledge, this N,N-chelating mode of
a N,P,N ligand in 2 and 3 is unprecedented and also results,
according to the CCDC, in the first transition metal complexes
with a N,P,N ligand forming an eight-membered ring. One
example has been reported with gallium25a and another theoretical
structure recently calculated for niobium.25b
The molecular structures of 2·0.5C7H8 and 3 indicate that
the occurrence of a N,N-chelating mode for a N,P,N ligand in
transition metal coordination chemistry may be more general than
originally considered and could play a significant role in explaining
the catalytic properties of such complexes.
For comparison, we reacted the ligand NPNMe2 with
one equiv. of [Pd(Me)Cl(COD)] (COD = 1,5-cyclooctadiene),
[PdCl2(NCPh)2] or [Pd(NCMe)4](BF4)2 in CH2Cl2 and obtained
complexes 4–6, respectively (see ESI§).
Fig. 2 ORTEP view of the molecular structure of 2 in 2·0.5C7H8. Ellip-
soids enclose 50% of the electron density. Hydrogen atoms omitted for clar-
◦
˚
ity. Selected bond distances [A] and angles [ ]: Co–Cl1 2.2447(10), Co–Cl2
2.2459(10), Co–N1 2.036(3), Co–N2 2.049(3); N1–Co–N2 108.8(1),
N1–Co1–Cl1 105.4(1), N2–Co1–Cl1 111.47(9), N1–Co1–Cl2 111.14(9),
N2–Co1–Cl2 108.54(9), Cl1–Co1–Cl2 111.53(4).
˚
The Co–P separation of 4.307(1) A is very long and the
phosphorus lone pair is not properly oriented to coordinate the
metal centre. The N1–Co and N2–Co distances of 2.036(3) and
˚
2.049(3) A respectively, are similar to those in tetrahedral cobalt
complexes coordinated by N,N-diimine ligands but the N1–Co–
N2 angle of 108.8(1)◦ is significantly larger.20,21
Iron complexes are also of current interest in olefin oligomer-
ization and polymerization catalysis where their performances
depend very much on the ligand used and may be very
significant.13,14,22–24 We thus reacted NPNMe2 with FeCl2 and
obtained a complex formulated as [FeCl2(NPNMe2)] 3. Its crystal
structure established that the N,P,N ligand again only functioned
as a N,N chelate (Fig. 3) as observed for 2 in 2·0.5C7H8. The
crystal structure of 3 shows a distorted tetrahedral geometry
around the Fe(II) centre. The N1–Fe and N2–Fe distances (2.088(2)
In contrast to the situation in 2 and 3, coordination of the
phosphorus atom to the metal centre is clearly indicated in the
1
31P{ H} NMR spectra by the shift from d −24.8 ppm for the free
ligand to 26.6 ppm for the coordinated ligand in 4 and 19.9 ppm
1
and 2.094(2) A, respectively) are slightly longer than the N1–
in 5. The 13C{ H} NMR spectra of 4 and 5 contain two signals
˚
2
=
Co and N2–Co distances found in 2·0.5C7H8. The separation of
for the C N oxazoline carbon: a doublet at 169.6 ppm ( JPC of
4.340(2) A between the Fe centre and the P atom rules out any
18.5 Hz, complex 4) or at 173.9 ppm (2JPC of 23.9 Hz, complex
5) corresponding to a coordinated oxazoline and a singlet at
159.4 ppm (complex 4) or a doublet at 158.2 ppm (2JPC of 7.6 Hz,
complex 5) corresponding to an uncoordinated oxazoline. The
˚
bonding interaction between these atoms. In both the structures
of 2·0.5C7H8 and of 3, the marked torsion of the N,P,N ligand is
noteworthy.
586 | Dalton Trans., 2008, 585–587
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1H NMR spectra confirm the presence of the coordinated and
uncoordinated oxazoline with two distinct ABX spin systems
(A = H, B = H, X = P) for PCHAHB and two distinct AB
collected using phi-scans and the structures were solved by direct methods
using the SHELX 97 software,26 and the refinement was by full-matrix
least squares on F2. No absorption correction was used. All non-hydrogen
atoms were refined anisotropically with H atoms introduced as fixed
A
B
=
spin systems for OCH H . Two m(C N) vibrations for the
uncoordinated and coordinated oxazolines appear in the IR
spectrum at 1659 and 1630 cm−1 for 4 and 1659 and 1614 cm−1 for
5, respectively.
˚
contributors (dC–H = 0.95 A, U11 = 0.04). Crystallographic data for
1·0.5C7H8. C22H33Cl2CoN2O4P·0.5(C7H9), M = 596.37, triclinic, space
¯
˚
˚
group P1, T = 173(2) K, a◦= 9.1010(4) A, b = 9.1930(4) A, c =
◦
◦
˚
17.5460(9) A, a = 79.9750(15) , b = 84.2710(15) , c = 83.173(3) , V =
3
˚
1430.62(11) A , Z = 2, reflections: measured/independent/observed =
10153/6620/4065, Rint = 0.044, R [F2 >2r(F2)] = 0.061, wR (F2) = 0.180,
S = 1.01.
1
The 31P{ H} NMR spectrum of 6 established again the coor-
dination of the phosphorus donor atom to the metal centre, in
Crystallographic data for 2·0.5C7H8. C18H25Cl2CoN2O2P·0.5(C7H9), M =
marked contrast with the situation encountered above with 2 and
˚
1015.53, monoclinic, space group P21/c, T = 173(2) K, a = 9.1785(2) A,
1
3. As expected and in contrast to 4 and 5, the 13C{ H} NMR
◦
3
˚
˚
˚
b = 12.1326(5) A, c = 23.4657(7) A, b = 111.606 (2) , V = 2429.51(13) A ,
Z = 2, reflections: measured/independent/observed = 14007/5541/3981,
Rint = 0.040, R [F2 >2r(F2)] = 0.058, wR (F2) = 0.175, S = 1.07.
=
spectrum of 6 presented only a doublet for the C N oxazoline at
174.7 ppm (2JPC = 19.8 Hz). Coordination of the two oxazoline
Crystallographic data for 3. C18H25Cl2FeN2O2P, M = 459.12, monoclinic,
1
moieties was also confirmed by H NMR spectroscopy with the
˚
˚
˚
space group P21/c, T = 173(2) K,◦a = 12.7240(13) A, b = 12.3190(15) A,
presence of only one ABX spin system for the PCHAHB protons
3
˚
c = 15.5250(19) A, b = 113.772(5) , V = 2227.0(4) A , Z = 4, reflections:
measured/independent/observed = 10818/6465/4953, Rint = 0.029, R [F2
>2r(F2)] = 0.057, wR (F2) = 0.162, S = 1.08.
and one AB spin system for the OCHAHB protons and by IR
=
spectroscopy with the presence of a unique band for the m(C N)
vibration at 1613 cm−1.
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784.
The ligands NOPONMe2 and NPNMe2 thus present a similar
coordination behaviour in their Pd(II) complexes and always
behave as P donor ligands. The tetrahedral geometry of the
cobalt and iron complexes does not readily explain why N,N-
coordination is so much favored in 2 and 3 instead of the
P,N-coordination observed in complex 1 and further work is in
progress.
8 F. Speiser, P. Braunstein and L. Saussine, Dalton Trans., 2004, 1539.
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Preliminary studies on catalytic ethylene oligomerisation
showed activities up to 17700 molC molCo−1 h−1 for 1 with 6 equiv.
H
2
4
of AlEtCl2 as cocatalyst under an ethylene pressure of 30 bar at
80 ◦C. Further catalytic tests are in progress.
This work was supported by the Centre National de la
Recherche Scientifique (CNRS), the Ministe`re de lꢁEducation
Nationale et de la Recherche (Paris) and the Institut Franc¸ais du
Pe´trole (IFP). We also thank Dr A. DeCian and Prof. R. Welter
(ULP Strasbourg) for the crystal structure determinations, Dr
R. Pattacini for discussions and Falk Tomicki (Erasmus-Socrates
exchange programme, University Duisburg-Essen) for preliminary
experiments.
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Notes and references
¶ 1: Anhydrous CoCl2 (0.14 g, 1.1 mmol) was added to a solution of
NOPONMe2 (0.46 g, 1.1 mmol) in 30 mL of THF. The blue solution was
stirred for 3 h at room temp. THF was removed under reduced pressure
and the blue powder was dried overnight under vacuum (yield: 0.58 g,
1.05 mmol, 97%). Data in ESI.§
2: Anhydrous CoCl2 (0.24 g, 1.85 mmol) was added to a solution of NPNMe2
(0.62 g, 1.85 mmol) in 50 mL of THF. The blue solution was stirred for 3 h at
room temp. After filtration of the solution and elimination of THF under
reduced pressure, the blue powder was dried overnight under vacuum
(yield: 0.81 g, 1.75 mmol, 94%). Data in ESI.§
3: To a solution of NPNMe2 (1.99 g, 6.0 mmol) in 50 mL of CH2Cl2 was
added a CH2Cl2 solution of FeCl2·4H2O (1.19 g, 6.0 mmol) and the mixture
was stirred at room temp. overnight. The solvent was then eliminated under
reduced pressure and 50 mL of diethyl ether was added to precipitate the
white complex which was recovered by filtration, washed with diethyl ether
and dried under vacuum (yield: 2.40 g, 5.2 mmol, 87%). Data in ESI.§
ꢀ Diffraction data were collected on a Kappa CCD diffractometer using
˚
graphite-monochromated Mo-Ka radiation (k = 0.71073 A). Data were
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 585–587 | 587
©