Combined experimental and theoretical study of bis(diphenylphosphino)(N -thioether)amine-type ligands in nickel(II) complexes for catalytic ethylene oligomerization

Article abstract of DOI:10.1021/om500141k

Starting from the new ligands bis(diphenylphosphino)(N-4-(methylthio) phenyl)amine (4, N(PPh₂)₂(p-C₆H ₄)SMe) and its monosulfide derivative (Ph₂P)N{P(S)Ph ₂}(p-C₆H₄)SMe (4·S), we have prepared and characterized, including by X-ray crystallographic studies, their Ni(II) complexes [NiCl₂{(Ph₂P)₂N(p-C₆H ₄)SMe-P,P}] (5) and [NiCl₂{(Ph₂P)N{P(S)Ph ₂}(p-C₆H₄)SMe-P,S}] (6), respectively. The bis-sulfide compound N{P(S)Ph₂}₂(p-C₆H ₄)SMe (4·S₂) was also prepared and structurally characterized. Computational studies showed that the combined influence of stronger P donors and a four-membered-ring P,P chelate leads to complex 5 being thermodynamically more stable than 6, which contains one weaker P-S donor group but a five-membered P,P-S chelate ring. For comparison, the bis-chelate complex [Ni{(Ph₂P)N{P(S)Ph₂}(p-C₆H₄)SMe-P,S} ₂](BF₄)₂ (7), the monochelate complexes [NiBr₂{(Ph₂P)N{P(S)Ph₂}(p-C₆H ₄)SMe-P,S}] (8) and the Pd(II) analogue of 6, [PdCl ₂{(Ph₂P)N{P(S)Ph₂}(p-C₆H ₄)SMe-P,S}] (9), were synthesized and structurally characterized and their solution behavior was investigated. The catalytic activity and selectivity in ethylene oligomerization of the Ni(II) complexes 5 and 6 and their known N-(methylthio)propyl analogues [NiCl₂{(Ph₂P) ₂N(CH₂)₃SMe-P,P}] (2) and [NiCl ₂{(Ph₂P)N{P(S)Ph₂}(CH₂) ₃SMe-P,S}] (3), which were obtained from the bis(diphenylphosphino) (N-(methylthio)propyl)amine ligand N(PPh₂)₂(CH ₂)₃SMe (1) and its monosulfide derivative (Ph ₂P)N{P(S)Ph₂}(CH₂)₃SMe (1·S), respectively, revealed a significant influence of the nature of the N-substituent (aryl vs alkyl thioether) and of the chelate ring size (P,P vs P,P-S). DFT calculations showed that the trend in ΔE_rel, [NiCl₂(P,P)] > [NiCl₂(P,P-S)] > [NiCl ₂(P-S,P-S)], results from the stronger covalent character of the Ni-P vs Ni-S bond. Using AlEtCl₂ as cocatalyst, mostly ethylene dimers were produced, with significant amounts of trimers (selectivity in the range 11-36%). Productivities up to 40400 and 48200 g of C₂H ₄/((g of Ni) h), with corresponding TOF values of 84800 and 101100 mol of C₂H₄/ ((mol of Ni) h), were obtained with precatalysts 2 and 3, respectively.

Full text of DOI:10.1021/om500141k

Article
pubs.acs.org/Organometallics
Combined Experimental and Theoretical Study of
Bis(diphenylphosphino)(N‑thioether)amine-Type Ligands in Nickel(II)
Complexes for Catalytic Ethylene Oligomerization
Alessio Ghisolfi, Christophe Fliedel,*,† Vitor Rosa,† Kirill Yu. Monakhov,‡ and Pierre Braunstein*
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Universite
Strasbourg Cedex, France
́
de Strasbourg, 4 rue Blaise Pascal, 67081
S
* Supporting Information
ABSTRACT: Starting from the new ligands bis-
(diphenylphosphino)(N-4-(methylthio)phenyl)amine (4, N-
(PPh2)2(p-C6H4)SMe) and its monosulfide derivative (Ph2P)N{P-
(S)Ph2}(p-C6H4)SMe (4·S), we have prepared and characterized,
including by X-ray crystallographic studies, their Ni(II) complexes
[NiCl2{(Ph2P)2N(p-C6H4)SMe-P,P}] (5) and [NiCl2{(Ph2P)N{P-
(S)Ph2}(p-C6H4)SMe-P,S}] (6), respectively. The bis-sulfide com-
pound N{P(S)Ph2}2(p-C6H4)SMe (4·S2) was also prepared and
structurally characterized. Computational studies showed that the
combined influence of stronger P donors and a four-membered-ring
P,P chelate leads to complex 5 being thermodynamically more stable
than 6, which contains one weaker PS donor group but a five-
membered P,PS chelate ring. For comparison, the bis-chelate
complex [Ni{(Ph2P)N{P(S)Ph2}(p-C6H4)SMe-P,S}2](BF4)2 (7), the
monochelate complexes [NiBr2{(Ph2P)N{P(S)Ph2}(p-C6H4)SMe-P,S}] (8) and the Pd(II) analogue of 6, [PdCl2{(Ph2P)N-
{P(S)Ph2}(p-C6H4)SMe-P,S}] (9), were synthesized and structurally characterized and their solution behavior was investigated.
The catalytic activity and selectivity in ethylene oligomerization of the Ni(II) complexes 5 and 6 and their known N-
(methylthio)propyl analogues [NiCl2{(Ph2P)2N(CH2)3SMe-P,P}] (2) and [NiCl2{(Ph2P)N{P(S)Ph2}(CH2)3SMe-P,S}] (3),
which were obtained from the bis(diphenylphosphino)(N-(methylthio)propyl)amine ligand N(PPh2)2(CH2)3SMe (1) and its
monosulfide derivative (Ph2P)N{P(S)Ph2}(CH2)3SMe (1·S), respectively, revealed a significant influence of the nature of the N-
substituent (aryl vs alkyl thioether) and of the chelate ring size (P,P vs P,PS). DFT calculations showed that the trend in ΔErel,
[NiCl2(P,P)] > [NiCl2(P,PS)] > [NiCl2(PS,PS)], results from the stronger covalent character of the Ni−P vs Ni−S
bond. Using AlEtCl2 as cocatalyst, mostly ethylene dimers were produced, with significant amounts of trimers (selectivity in the
range 11−36%). Productivities up to 40400 and 48200 g of C2H4/((g of Ni) h), with corresponding TOF values of 84800 and
101100 mol of C2H4/ ((mol of Ni) h), were obtained with precatalysts 2 and 3, respectively.
membered-ring nickel(II) complexes with chelating short-bite
ligands have also been successfully used as precatalysts in
ethylene oligo- or polymerization, although they do not display
the selectivity of the analogous Cr complexes.7 The presence of
a cocatalyst, such as an organoaluminum compound, is
generally required and the characterization of the active species
formed remains challenging.8 To date, no Ni(PNP)/Al system
could be isolated, and only one bimetallic Cr(II)/Al complex
supported by a DPPA-type ligand, resulting from the reaction
of [CrCl3{(Ph2P)2N(Cy)-P,P}] with AlMe3 as activator, could
be structurally characterized.9
INTRODUCTION
■
Short-bite ligands, such as bis(diphenylphosphino)methane-
type (DPPM) and bis(diphenylphosphino)amine-type (DPPA)
ligands, offer a diversity of coordination modes (monodentate,
bidentate chelating, or bridging), which is reflected in their rich
coordination chemistry. While their neutral forms,
Ph2PCH2PPh2 and Ph2PNHPPh2, respectively, act as four-
electron donors on coordination to a metal center through both
phosphines, an additional X-type donor function is available in
their deprotonated forms (Ph2PCHPPh2)− and (Ph2PNPPh2)−,
respectively.1 Furthermore, various N-substituents can be
readily introduced in DPPA-type ligands, which may facilitate
the formation of polymetallic complexes2 or the anchoring of
coordination compounds into mesoporous materials3 or on
metallic surfaces.4 Metal complexes of such short-bite ligands
are involved in various catalytic reactions,5 and chromium
derivatives in particular have been intensely studied for the
selective catalytic tri- and tetramerization of ethylene.6 Four-
Five- and six-membered-ring Ni(II) complexes have been
successfully applied to ethylene oligomerization, and typical
examples are shown in Chart 1.10−12
Received: February 7, 2014
Published: May 6, 2014
© 2014 American Chemical Society
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Chart 1. Examples of “Post-Metallocene” Nickel Catalysts
for Olefin Oligo-/Polymerization10−12
Scheme 1. Bis(diphenylphosphino)(N-
(methylthio)propyl)amine Ligand 1 and Its Chelate
Complex [NiCl2(P,P)] (2) (Top) and Equilibrium between
the Neutral Diamagnetic Complex [NiCl2(P,PS)] (3) and
the Paramagnetic Ion Pair [Ni(P,PS)2][NiCl4] (3′)
Containing the Monosulfide Derivative Ligand 1·S (Bottom)
Our interest in catalytic ethylene oligomerization has led us
inter alia to prepare various Ni complexes with mixed P,L
ligands (L = N, O) (Chart 2).13 The association within a
functional ligand of donor sets of different hard−soft properties
leads to so-called hybrid ligands,14 which have been of
longstanding interest to us.14 Hybrid ligands bring about
a,b
stereoelectronic differentiation within the metal coordination
sphere, in particular in the positions trans to these donor atoms.
Their metal complexes can exhibit in solution a hemilabile
behavior resulting from the reversible coordination−decoordi-
nation of the weaker donor. This feature allows stabilization of
potentially coordinatively unsaturated species and facilitates
substrate coordination for improved catalytic performance.14
Starting from short-bite ligands, one can readily obtain a
hybrid ligand by selective mono-oxidation of one P-donor
group into a phosphorus oxide or sulfide, which can potentially
form a five-membered-ring chelate upon metal coordination.
Oxidation of both P(III) centers would of course restore a
symmetrical bidentate ligand, suitable for the formation of six-
membered-ring metal complexes. Starting from bis-
(diphenylphosphino)(N-(methylthio)propyl)amine (1, N-
(PPh2)2(CH2)3SMe), we recently prepared its monosulfide
(Ph2P)N{P(S)Ph2}(CH2)3SMe (1·S) and obtained their
corresponding Ni(II) complexes [NiCl2{(Ph2P)2N-
(CH2)3SMe-P,P}] (2) and [NiCl2{(Ph2P)N{P(S)Ph2}-
(CH2)3SMe-P,S}] (3), which contain a P,P (2) or a P,PS
(3) chelate, respectively. These complexes exhibit very different
reactivities in solution.15 In contrast to the symmetrical chelate
complex [NiCl2(P,P)] (2), which is stable in solution at various
temperatures and in the presence of MeOH, a solvent-
dependent reversible ligand exchange was found to be
remarkably easy in the unsymmetrical chelate complex
[NiCl2(P,PS)] (3) (Scheme 1).
(P,PS)2][NiCl4] (3′), which involves the reversible transfer
of the chelating ligand 1·S, accompanied by chloride migration.
This equilibrium was studied by multinuclear NMR, UV−
visible, and FTIR spectroscopic methods and density functional
theory (DFT) calculations.15 After identifying the factors
responsible for the equilibrium, we have isolated, characterized,
and produced selectively one isomer or the other.
The coordination chemistry of the Ni(II) ion with other
hybrid ligands, of the P,N type, was recently shown to lead to
high versatility and facile structural modifications.16 These
included (i) the pressure-induced fragmentation of a
tetranuclear complex with 20-electron Ni centers into
mononuclear complexes [NiCl2(P,N)] with 16-electron Ni
centers,17 (ii) the formation of mono- or dinuclear isomers as a
function of the crystallization temperature,13a,18 and (iii) the
equilibrium in solution between the two formula isomers
[Ni(P,N)2][NiCl4] and [NiCl2(P,N)].19
Starting from short-bite ligands, we were interested in
studying how small changes in the metal catalyst design may
modify their catalytic properties. We examined first the
influence of the N-substituent in DPPA-type ligands (type 1
vs type 4 ligands with a flexible or more rigid tail, respectively)
and then that of the size of the chelate ring in the complex,
An equilibrium was established in CH2Cl2 at room
temperature between the neutral diamagnetic complex
[NiCl2(P,PS)] (3) and the paramagnetic ion pair [Ni-
Chart 2. Examples of Ni(II) Precatalysts with Phosphorus-Based P,L Chelates for Ethylene Oligomerization from Our
Laboratory13
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although we realize that this is accompanied by a change in the
nature of one of the donor groups (four-membered ring in P,P
chelates vs five-membered ring in the corresponding P,PS
chelates). One objective was to provide unambiguous
characterization, preferably both in the solid state and in
solution, of precatalysts to be used in ethylene oligomerization.
We report here the synthesis, characterization, and DFT
calculations of the Ni(II) complexes [NiCl2{(Ph2P)2N(p-
C6H4SMe)-P,P}] (5) and [NiCl2{(Ph2P)N{P(S)Ph2}(p-
C6H4SMe)-P,S}] (6), prepared from the ligand bis-
(diphenylphosphino)(N-4-(methylthio)phenyl)amine (4) and
its monosulfide derivative 4·S, respectively. These complexes
are N-aryl thioether analogues of complexes 2 and 3 obtained
with N-alkyl thioether DPPA-type ligands. We also discuss the
catalytic activities of the corresponding NiCl2 complexes (2, 3
and 5, 6) in ethylene oligomerization, with ethylaluminum
dichloride as cocatalyst, as a function of the influence of the N-
substituent (aryl vs alkyl function) in the chelating diphosphine
ligand and of the size of the chelate ring formed upon
coordination to the Ni(II) center.
phosphorus atoms (Table 1). This chemical shift is in
agreement with the values observed for N-aryl-substituted
DPPA-type ligands (62.2−71.1 ppm), higher values being
found for compounds with aryl groups substituted at the para
position (66.1−71.1 ppm).21 The monosulfide derivative 4·S
could not be obtained in pure form, owing to the concomitant
formation of the bis-sulfide byproduct 4·S2 in a ca. 70:30 ratio
(Scheme 2, reactions b and c)). A similar situation was observed
in the case of ligand 1·S, which like 4·S was prepared and used
in situ for the synthesis of the desired Ni(II) complexes. The
undesired compound 4·S2 can easily be removed by successive
washings of its Ni(II) complexes with toluene (see below).
However, the bis-sulfide derivative 4·S2 can be quantitatively
obtained, as an air-stable white powder, using a slight excess of
elemental sulfur, but its coordination properties will not be
discussed here (Scheme 2, reaction c). Both 4·S and 4·S2 were
fully characterized, and their 31P{1H} NMR spectra exhibit the
expected patterns: i.e., two doublets (55.3 (PIII) and 72.4 (PV)
2
ppm; JP,P = 104.5 Hz) for the mixed P,PS ligand 4·S and a
singlet (69.0 ppm) for the two equivalent phosphorus atoms of
4·S2 (Table 1). These chemical shifts are comparable to those
reported for other mono- and bis-sulfide DPPA-type ligands,
which were found in the respective ranges: 51.5−54.5 (PIII) and
67.5−73.5 (PV)21g,h,22 (monosulfide derivatives) and 67.5−73.6
ppm (bis-sulfide derivatives).21a−d,g,23 The experimental
31P{1H} NMR chemical shifts of the N-thioether-functionalized
P,PS and PS,PS ligands (1·S, 1·S2, 4·S, and 4·S2)
measured in CDCl3 and CD2Cl2 solutions were well
reproduced by NMR calculations on the BP86/TZ2P
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization of the
N-Arylthioether DPPA-Type Ligands. The preparation of
bis(diphenylphosphino)(N-4-(methylthio)phenyl)amine (4)
was readily achieved by reaction of the corresponding amines
with 2 equiv of PPh2Cl in the presence of excess NEt3 as HCl
scavenger (Scheme 2, reaction a).20
Scheme 2. Synthesis of (P,P), (P,PS), and (PS,PS)
minimum-energy gas-phase structures of 1·SDFT, 1·S2DFT, 4·
a
Ligands 4, 4·S, and 4·S2
DFT
S
DFT, and 4·S2
(Supporting Information, Tables S4−S8).
The molecular structures of ligands 4 and 4·S2 were
determined by single-crystal X-ray diffraction studies and are
depicted in Figures 1 and 2, respectively. To the best of our
knowledge, only four other bis(diarylphosphino)(N-4-(R)-
phenyl)amine ligands have been structurally characterized
(A−D),3a,21a,24,25 and interesting structural differences with
ligand 4 were observed (Scheme 3 and Figure S-1 (Supporting
Information)).
While in the reported compounds A,3a B,24 and C,21a as well
as in ligand 1,20 the P lone pairs display a syn conformation
(pointing in the same direction, a typical chelating coordination
orientation), those of ligand 4 and the P-benzyl derivative D25
present an anti orientation (pointing in opposite directions;
Scheme 3, bottom). The structure of 4 also exhibited a stronger
tilt of the angle (α) between the aryl ring and the PNP plane (α
= 62.1°) in comparison to A (α = 82.4°), B (α = 80.5°), C (α =
81.7°), and D (α = 78.3°) (Scheme 3, top). In view of the
structures of ligands ADFT (α = 84.4°), BDFT (α = 83.5°), and
4DFT (α = 61.7°) calculated in the gas phase, one can say that
the conformational difference associated with the α angle is
rather “intrinsic” to 4 instead of being caused by packing effects
(Supporting Information, Table S9). The other possible
a
Legend; (a) PPh2Cl (2 equiv), NEt3 (2.5 equiv), THF, room
temperature, 3 h; (b) S8 (1.5 equiv), CH2Cl2, room temperature, 30
min; (c) S8 (2.2 equiv), toluene, 115 °C, 3 h.
The ligand 4 was obtained in good yield (71%) as an air- and
moisture-sensitive white powder. Its 31P{1H} NMR spectrum
contains a singlet resonance at 69.4 ppm for the two equivalent
Table 1. 31P{1H} NMR Data of Ligands 4, 4·S2, and 4·S and Complexes 5−9
a
a
a
b
a
b
ad
,
ad
,
4
4·S2
4·S
5
6
7
8
9
δ (ppm)
mult
69.4
s
69.0
s
55.3; 72.4
d; d
2JP,P = 104.5
45.4
s
83.8
br s
84.6; 105.8
t; t
83.7
br s
80.1; 106.7
d; d
c
2,2+3,3,4
2+3
J (Hz)
J
P,P
J
P,P
= 59.9
a
b
c
d
NMR solvent CDCl3. NMR solvent CD2Cl2. Detailed in the Experimental Section. See the Supporting Information.
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Scheme 3. Main Structural Differences between Ligand 4
and the known Ligands A−D
The solid-state structure of 4·S2 revealed characteristic PS,
P−N, and C−N bond lengths and a P−N−P angle in the usual
range.21c,22d,23a,26 All attempts to obtain suitable crystals of
ligand 4·S for X-ray diffraction were unsuccessful, which may
explain the relatively small number of structures of mono-
sulfide-DPPA derivatives reported.22d,e Interestingly, the
oxidation of the P centers and the formation of the bis-sulfide
ligand 4·S2 result in only slight structural differences in
comparison to 4. The P−N and C−N bond lengths are almost
similar (average 1.73 Å (4) vs 1.71 Å (4·S2)), while the most
affected parameters are the PNP angle (120.8(1)° (4) vs
127.1(7)° (4·S2)) and the angle between the PNP moiety27 and
the phenyl ring (α = 62.1° (4) vs. 55.6° (4·S2)) (Scheme 3).
The BP86/TZ2P calculations provide values for the angles α
comparable to those determined experimentally. Thus, the α
angle between the planes of the PNP moiety and the N-aryl
ring was calculated to be 61.7° for 4DFT, 57.6° for 4·S2DFT, and
79.3° for the hypothetical structure 4·SDFT. The calculated PNP
angles in 4DFT, 4·S2DFT, and 4·SDFT are 120.8, 129.0, and 117.2°,
respectively. As a result, a correlation between the α and PNP
Figure 1. ORTEP drawing of the molecular structure of N(PPh2)2(p-
C6H4)SMe (4). Thermal ellipsoids are set at the 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond distances (Å)
and angles (deg): P1−N1 1.732(2), P2−N1 1.728(2), C1−N1
1.452(3); P1−N1−P2 120.8(1).
angles was observed: the greater the α angle (4·S2DFT < 4DFT
<
4·SDFT), the smaller the PNP angle (4·S2DFT > 4DFT > 4·SDFT).
Synthesis of the Ni(II) Complexes. With the objective of
developing new (pre)catalysts for ethylene oligomerization and
evaluating the influence of the ligand properties (P,P vs P,PS,
chelate ring size, nature of the N-substituent), we prepared
nickel(II) complexes with ligands 4 and 4·S by analogy with
complexes of the type [NiX2(LL′)] (e.g. LL′ = P,N, P,O, or P,P
ligand and X = Cl, Br).13a,b,28 Reaction of 4 with
[NiCl2(DME)] (DME = 1,2-dimethoxyethane) in a 1:1 ratio
in dichloromethane at room temperature afforded
[NiCl2{(Ph2P)2N(p-C6H4SMe)-P,P}] (5) in high yield (83%)
as a red crystalline powder (Scheme 4, reaction a). This color is
consistent with the square-planar structure and diamagnetic
nature of Ni(II) complex 5, which could therefore be fully
characterized by multinuclear NMR spectroscopic methods. Its
31P{1H} NMR spectrum exhibits a singlet at 45.4 ppm, shifted
upfield with respect to the free ligand 4 (s, 69.4 ppm, Table 1).
This value compares well with those found for the other
[NiX2(P,P)] (X = Cl, Br, I and P,P = N-functionalized DPPA-
type ligand) complexes reported (38.6−58.3 ppm).7b,c,e,15,23c,29
Figure 2. ORTEP drawing of the molecular structure of N{P(S)-
Ph2}2(p-C6H4)SMe (4·S2). Thermal ellipsoids are set at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected
bond distances (Å) and angles (deg): P1−N1 1.708(1), P2−N1
1.717(1), P1−S1 1.9452(6), P2−S2 1.9350(6), N1−C25 1.457(2);
P1−N1−P2 127.11(7), S1−P1−N1 115.16(5), S2−P2−N1
114.82(5).
relationships between the orientation of P lone pairs and the α
angle were not explored further.
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a
1298.99, which can be attributed to {[NiCl2(4·S)] − Cl}+ and
{[Ni(4·S)2][NiCl4] − Cl}+, respectively. Unfortunately, direct
comparison of NMR data with literature values was not
possible since, as far as we know, no other NiCl2 complex of
monosulfide N-functionalized DPPA-type ligands has been
reported. Only PdCl2 and PtCl2 complexes have been
described, which of course exhibit well-resolved signals, such
as [PdCl2(4·S)] (9) (see Table 1 and the Supporting
Information).21g,h,22b,c,e It is worth noting that no change in
the 31P{1H} NMR spectrum of 9 was observed, even by
modification of the solvent polarity or recording temperature,
in contrast to its Ni(II) analogues 6 and 8, reflecting the higher
kinetic stability in solution of Pd derivatives. While isomer 6
could be isolated in pure form, all attempts to isolate the
hypothetical isomer 6′ by increasing the solvent polarity upon
addition of MeOH (as for 3′)15 led to a metal-induced
dismutation-type reaction of the phosphorus atoms, resulting in
the formation of the non-PS-containing complex 5 and the
bis-sulfide ligand 4·S2 in a 1:1 ratio (NMR evidence). This
transformation, from 6 to 5 and 4·S2, was also observed when
pure 6 was dissolved d6-DMSO, an aprotic but polar solvent.
The latter characteristic thus appears decisive for the
occurrence of this ligand transformation.
Scheme 4. Synthesis of the Ni Complexes 5−7
a
Legend: (a) 1 equiv of [NiCl2(DME)], CH2Cl2, room temperature, 4
h; (b) 1.5 equiv of S8, CH2Cl2, room temperature, 30 min and 0.55
equiv of [NiCl2(DME)], CH2Cl2, room temperature, 4 h; (c) 1.5
equiv. of S8, CH2Cl2, room temperature, 30 min followed by 0.25
equiv of [Ni(NCMe)6][BF4]2, CH2Cl2, room temperature, 4 h. Note:
complexes 6 and 6’ are in equilibrium in solution. DME = 1,2-
dimethoxyethane.
Since the paramagnetic component of 6′ is [NiCl4]2−, we
prepared the diamagnetic bis-chelated complex [Ni{(Ph2P)N-
{P(S)Ph2}(p-C6H4)SMe-P,S}2](BF4)2 (7), containing the same
dication as the hypothetical complex 6′, using [Ni(NCMe)6]-
[BF4]2 as precursor instead of [NiCl2(DME)] (Scheme 4,
reaction c). Complex 7 was obtained in good yield (85%) as a
crystalline solid, its red color (yellow in diluted solution) being
characteristic of a square-planar arrangement of the ligands
around the metal center. Its diamagnetic nature therefore
allowed its full characterization by multinuclear NMR experi-
ments, and its 31P{1H} NMR spectrum exhibited two tripletlike
resonances (AA′BB′ spin system) at 105.8 (PV) and 84.6 (PIII)
ppm (Table 1).
Since 4·S could not be isolated in pure form, it was prepared
in situ (see above), and subsequent addition of 0.5 equiv of
[NiCl2(DME)] led to the formation of [NiCl2{(Ph2P)N{P-
(S)Ph2}(p-C6H4SMe)-P,S}] (6) in good yield (82%) as a
crystalline solid, after removal of the undesired 4·S2 by
successive washings with toluene (Scheme 4, reaction b). Its
purple color, both in the solid state and in solution, contrasts
with that of the diamagnetic complex 5 (red). The 31P{1H}
NMR spectrum of a solution of 6 exhibited a broad singlet
centered at 83.8 ppm (Table 1) instead of the expected pattern
of two doublets for the nonequivalent PIII and PV donors, as
observed for the free ligand. A similar 31P{1H} NMR spectrum
was obtained for the complex [NiBr2(4·S)] (8), which suggests
no significant influence of the halide nature on the solution
behavior of such P,PS-chelate Ni(II) complexes (see Table 1
and the Supporting Information). The paramagnetic character
of the solution prevented the recording of well-resolved 1H and
13C NMR spectra. We recently showed in the case of complex 3
(Scheme 1), which is the analogue of 6 with the N-alkyl
thioether ligand 1·S, that this broad signal observed in the
NMR is due to a fast equilibrium in solution between the
neutral diamagnetic [NiCl2(1·S)] (3) species and its para-
magnetic ion pair formula isomer [Ni(1·S)2][NiCl4] (3′).15
This equilibrium was found to be dependent on the
temperature and the solvent polarity and could be unambig-
uously characterized by 31P VT-NMR and UV−vis (variable
temperature and polarity gradient) spectroscopic techniques
and rationalized by computational studies. On the basis of this
previous work, we reasonably assume that a similar equilibrium
between the anticipated [NiCl2(4·S)] structure and the ion pair
formula isomer [Ni(4·S)2][NiCl4] (6′) (Scheme 4), as well as
for its NiBr2 analogue 8, can be envisaged. Furthermore, the
ESI-MS spectrum of a CH2Cl2 purple solution of complex 6
confirmed this hypothesis by revealing peaks at m/z 632.01 and
An NMR experiment consisting of the addition of Cl− anions
(from HNEt3Cl) to a solution of 7 showed an evolution of the
NMR spectrum, from two well-resolved “triplets” to one broad
singlet centered at 83.8 ppm, similar to that of 6, and the
liberation of uncoordinated ligand 4·S (see the Experimental
Section and Figure S2 (Supporting Information)). This
reaction is accompanied by a significant color change from
yellow (7) to the familiar purple of complex 6. These
observations are reminiscent of the behavior of analogous
complexes containing the N-alkyl thioether ligand 1·S (Scheme
1).15 From the unambiguously characterized equilibrium
between 3 and 3′, we conclude the existence of a similar
process between 6 and 6′.
The chemical shifts observed for the methyl protons in
complexes 5 and 7 (2.32 and 2.26 ppm, respectively) remain
very close to that for the free diphosphines 4 and 4·S (2.35 and
2.23 ppm, respectively), indicating no S···Ni interaction in
solution.
Solid-State Structure of the Ni(II) Complexes. Single
crystals of complexes 5−7 suitable for X-ray diffraction studies
were obtained as described in the Experimental Section. Their
solid-state structures are depicted in Figures 3−5, and selected
bond lengths and angles are reported in Table 2. In all three
complexes, the Ni center displays a distorted-square-planar
coordination geometry, with the P,P (4) and P,PS (4·S)
ligands acting as chelates, forming four- (5) and five-membered
(6, 7) metallacycles. The metal coordination sphere in
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Figure 3. ORTEP drawing of the molecular structure of
[NiCl2{(Ph2P)2N(p-C6H4)SMe-P,P}] (5) in 5·2CH2Cl2. Thermal
ellipsoids are set at the 50% probability level. Hydrogen atoms and
solvent molecules are omitted for clarity.
Figure 5. ORTEP drawing of the molecular structure of [Ni{(Ph2P)-
N{P(S)Ph2}(p-C6H4)SMe-P,S}2](BF4)2 (7) in 7·4CH2Cl2. Thermal
ellipsoids are set at the 50% probability level. Hydrogen atoms, BF4
anions, and solvent molecules are omitted for clarity.
complexes 5 and 6 is completed by two chlorine ligands, while
in the centrosymmetric structure of 7, a second P,PS ligand is
found in an anti arrangement.
The characteristic bond lengths and angles in 5 are similar to
those found in related [NiCl2(P,P)] (P,P = N-functionalized
DPPA-type ligand) complexes.6c,29b,e,30 With the exception of
complex 3, which was recently described,15 no other example of
a [NiCl2(P,PS)] complex is available for comparison, to the
best of our knowledge.
Interestingly, while the two N−P bond lengths are similar in
the (P,P)-Ni(II) complex 5, the difference between the N1−
P1III and the N1−P2V distances in the (P,PS)-Ni(II)
complexes 6 and 7 is more pronounced (1.731(3) and
1.678(3) Å (6) and 1.716(1) and 1.680(1) Å (7), respectively).
This could be related to the electronegativity of the sulfur atom,
which strengthens the P−N bond, as observed in related
complexes with PS, PO, and PSe groups.15,21g,22,22e,31
The P−N−P angle of the ligand is strongly affected by the
presence of the sulfur donor (96.7° (5) vs 118.0 (6) and 114.7°
(7)). In the [NiCl2(P,P)] complex 5, the rectangular distortion
of the coordination geometry around the metal is due to the
four-membered chelate ring, which imposes a P−Ni−P angle of
74.2°, while in the [Ni(P,PS)]-type complexes, the P−Ni−S
angle is closer to the preferred 90° (94.3° (6), 93.2° (7)).
Figure 4. ORTEP drawing of the molecular structure of [NiCl2{(Ph2P)N{P(S)Ph2}(p-C6H4)SMe-P,S}] (6) in 6·C3H6O. Thermal ellipsoids are set
at the 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity.
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(Figure 6). The trend in ΔErel is thus as follows: [NiCl2(P,P)] >
[NiCl2(P,PS)] > [NiCl2(PS,PS)]. From these data, we
may expect that the overall stability of the compounds, which in
the case of [NiCl2(P,PS)] is the combined result of a
modification in the chelate ring size (five-membered chelates
are usually more stable than four-membered chelates) and the
associated change in the nature of the donor groups (a PS
moiety is a poorer donor for Ni(II) than P) may influence the
catalytic properties of the Ni(II) complexes in ethylene
oligomerization.
Catalytic Ethylene Oligomerization. The nickel(II)
complexes with N-(methylthio)propyl (2, 3) or N-4-
(methylthio)phenyl (5, 6) functionalized PNP or P(S)NP-
type ligands, containing a four- (2, 5) or five-membered chelate
(3, 6), were evaluated as precatalysts in the oligomerization of
ethylene with AlEtCl2 as cocatalyst. Various amounts of
cocatalyst and different concentrations of precatalysts were
tested, and the catalytic results are presented in Table 3. To
allow comparisons with previous studies, we kept the reaction
time constant at 35 min and the ethylene consumption during
catalysis progressively decreased but was retained up to at least
60 min (entries 2 and 9, Table 3). The reaction products did
not follow a Schulz−Flory distribution of their chain length,
since mostly dimers and significant amounts of trimers
(selectivity in the range 11−36%) were obtained. The
percentages of C8 and C10 measured are in the ranges 0−
3.4% and 0−2.7%, respectively, and no longer chains were
observed.
Table 2. Selected Bond Lengths (Å) and Angles (deg) in the
Solid-State Structures of 5·2CH2Cl2, 6·C3H6O, and 7·
4CH2Cl2
5
6
7
N1−P1
N1−P2
P2−S1
P1−Ni1
P2(S1)−Ni1
Ni1−Cl1
Ni1−Cl2
1.706(5)
1.716(5)
1.731(3)
1.678(3)
2.004(1)
2.138(1)
2.163(1)
2.173(1)
2.211(1)
1.716(1)
1.680(1)
2.020(4)
2.198(3)
2.180(3)
2.110(2)
2.128(2)
2.183(2)
2.202(2)
Cl1−Ni1−Cl2
P1−Ni1−P2(S1)
P1−N1−P2
99.26(6)
74.20(6)
96.70(2)
94.91(4)
94.26(4)
118.0 (2)
93.16(1)
114. 7(6)
However, the Cl−Ni−Cl angle is only slightly affected (99.3°
(5) vs. 94.9° (6)).
As expected, the two P−Ni bond lengths in complex 5 are
nearly equivalent (average 2.12 Å) and similar to the P1−Ni
bond (2.14 Å) in complex 6. The slightly stronger trans
influence of the P donor compared to S in the mixed (P,PS)
nickel complex 6, results in a longer Ni−Cl2 bond compared to
Ni−Cl1, as also observed in related PdCl2 and PtCl2 complexes
with (P,PS) or (P,PSe) DPPA-type ligands.21g,22c,e In the
bis-chelated dicationic complex of 7, both L−Ni bonds (L = P,
S) were found to be 0.2−0.6 Å longer than in complex 6. The
solid-state molecular structures of [NiBr2(4·S)] (8·CH2Cl2)
and [PdCl2(4·S)] (9) are very similar to that of complex 6 (see
the Supporting Information, Figure S3 and Table S3).
The DFT relative energies (ΔErel) calculated for the reaction
processes associated with the formation of the Ni(II)
complexes of the N-alkyl thioether functionalized DPPA-type
ligands 1DFT, 1·SDFT, and 1·S2DFT (type A) and those of their N-
aryl thioether counter parts 4DFT, 4·SDFT, and 4·S2DFT (type B)
show that the presence of sulfur atoms at P significantly
influences the energetics of the nickel−ligand bonding, thus
resulting in decreased thermodynamic stability of the optimized
Ni(II) complexes with the (P,PS) and (PS,PS) ligands
Comparison between [NiCl2(P,P)]- and [NiCl2(P,PS)]-
Type Complexes. Such a comparison, made with a constant N-
substituent, allows us to examine the combined effect of a
change in the chelate ring size and of the resulting difference in
the nature of the donor atoms on the catalytic activity and/or
the selectivity. In the N-(methylthio)propyl series, the activity
of the P,PS complex 3 was found to be higher than that of its
P,P counterpart 2 at high Al/Ni ratios (10 and 40, entries 1−3
and 6, 7, respectively), with turnover frequencies (TOFs) of
47400 and 101100 mol of C2H4/((mol of Ni) h) for 3 and
35800 and 84800 mol of C2H4/((mol of Ni) h) for 2,
Figure 6. BP86/TZ2P relative energies (ΔErel in kJ mol−1, including zero-point energy corrections) calculated for the gas-phase reaction processes
involving minimum energy structures of the ligands analyzed herein and their Ni(II) complexes of types A and B. Color code: C, gray; N, blue; P,
orange; S, yellow; Cl, light green; Ni, dark green. For more details on the computational methodology, see the Supporting Information.
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Table 3. Pairwise Comparison of the Catalytic Data for Complexes 2, 3, 5, and 6 in the Oligomerization of Ethylene with
a
AlEtCl2 as Cocatalyst
selectivity (%)
bd
,
cd
,
e
entry no.
1
complex
ncomplex (10−5 mol)
AlEtCl2 (equiv)
C4
C6
C8
>C8
productivity
TOF
1-butene (%)
2
2
3
2
3
2
3
5
5
6
5
6
5
6
4
2
4
4
4
1
1
4
2
4
4
4
1
1
10
10
10
3
77
64
71
77
70
70
66
69
68
82
58
87
79
73
20
31
26
21
21
26
26
27
26
16
36
11
20
23
2
4
2
2
7
3
5
3
3
1
5
1
1
3
1
1
17100
23000
22600
3100
35800
48300
47400
6600
17
8.4
7
f
2
3
4
5
6
7
8
<1
0
16
35
16
25
4
3
2
2300
4800
40
40
10
10
10
3
1
40400
48200
23100
32800
12700
12900
3900
84800
101100
48400
68800
26700
27100
8200
2
<1
2
f
9
9
10
11
12
13
14
<1
1
9
9
3
<1
0
15
26
13
40
20000
41900
40
1
32200
67400
a
Conditions: T = 28−30 °C, 10 bar of C2H4, time 35 min, solvent 10 mL chlorobenzene and 5 mL of the cocatalyst solution (8 × 10−5 M) for 10
b
and 40 equiv of AlEtCl2 or 13.5 mL of chlorobenzene and 1.5 mL of the cocatalyst solution (8 × 10−5 M) for 3 equiv of AlEtCl2. In g of C2H4/((g
of Ni) h)). In mol of C2H4/((mol of Ni) h). Values rounded off to the lower hundred. Within the C4 fraction. Reaction time 60 min.
c
d
e
f
respectively. The chain-length selectivity was only slightly
affected on going from 2 and 3 (entries 1−7), but a higher
selectivity for C4 oligomers was observed for the P,P derivative
2 at all precatalyst concentrations and Al/Ni ratios examined.
Although modest, the best selectivity for 1-butene within the C4
cut was achieved with complex 3, using 3 equiv of AlEtCl2 and
4 × 10−5 mol catalyst loading and reached 35% (entry 5).
In the N-4-(methylthio)phenyl series, complex 6 exhibited
TOF values between 8200 and 26700 mol of C2H4/((mol of
Ni) h), while for 5, values between 27100 and 48400 mol of
C2H4/((mol of Ni) h) (entries 12 and 10 (6) and entries 11
and 8 (5), respectively) were obtained. At higher Al/Ni ratios,
the trend was reversed, since with 40 equiv of AlEtCl2 and 1 ×
10−5 mol catalyst loading, complex 6 exhibited a higher TOF
(67400 vs 41900 mol of C2H4/((mol of Ni) h)) than 5. The
selectivity in C4 oligomers was generally improved, with 3 or 10
equiv of AlEtCl2, by the introduction of the S donor and the
associated increase in the chelating ligand ring size, since it
reached 58 and 69% (entries 11 and 8) for complex 5 and 87
and 82% (entries 12 and 10), for complex 6, respectively. The
lower activity of 6 in comparison to that of 5 was associated
with an higher selectivity in C4 oligomers (73 vs 79%) (entries
14 and 13).
Influence of the N-Substituent. Let us now compare the
results obtained as a function of the N-substituent of the ligand,
keeping the chelate ring size constant: i.e., comparing 2/5 and
3/6. While complex 2 (P,P; N-alkyl thioether) showed similar
C4 selectivities when different amounts of Ni and Al cocatalyst
were used (70−77%, entries 1, 4, and 6), the C4 selectivity
observed for complex 5 (P,P; N-aryl thioether) varied much
more (58−79%, entries 8, 11, 13) and remained generally lower
than for 2. A greater difference in selectivity was observed for
the P,PS-containing Ni(II) complexes 3 and 6, and the latter
always exhibited a higher selectivity, up to 87% for C4
oligomers (entry 12), than its N-alkyl thioether counterparts
3 (70% under the same conditions, entry 5).
8, respectively) and 6600 (3 equiv of AlEtCl2) and 35800 (10
equiv of AlEtCl2) mol of C2H4/((mol of Ni) h) (entries 4 and
1), respectively. However, at higher Al/Ni ratios, using 40 equiv
of AlEtCl2, complex 2 led to TOF values more than twice those
of 5 (84800 vs 41900 mol of C2H4/((mol of Ni) h), entries 6
and 13, respectively). For the P,PS systems, however, the N-
alkyl complex 3 was found to be more active than its N-aryl
analogue 6 at high Al/Ni ratios (10 and 40 equiv of AlEtCl2,
entries 3 and 7 vs entries 10 and 14), and a lower TOF was
observed for 3 (4800 mol of C2H4/((mol of Ni) h), entry 4) in
comparison to 6 (8200 mol of C2H4/((mol of Ni) h), entry 12)
when only 3 equiv of AlEtCl2 was used.
Under the different conditions studied, the selectivity for α-
olefins remained modest (4−35%) due to the typical fast
isomerization process in Ni(II)-catalyzed oligomerization of
ethylene.11c,32 No saturated hydrocarbons were produced
during the catalytic process. The C6 and C8 oligomers
produced contained a small percentage of linear chains in a
larger distribution of branched olefins, which is indicative of the
occurrence of both stepwise chain growth and of codimeriza-
tion reactions (C4 + ethylene).
CONCLUSION
■
The selective oligomerization of ethylene remains an active
field of research, and the present work highlights, via a
combination of experimental and computational studies, how
subtle changes in the ligand design of the well-known (PNP)-
NiCl2 (pre)catalyst strongly influence their catalytic activity and
selectivity. Our synthesis and characterization of the new
bis(diphenylphosphino)(N-4-(methylthio)phenyl)amine ligand
4, its monosulfide derivative 4·S, and their Ni(II) complexes 5
and 6 follow our recent report on similar NiCl2 derivatives (2
and 3) of the bis(diphenylphosphino)(N-(methylthio)propyl)-
amine ligand 1 and its monosulfide derivative 1·S.15 The DFT
geometry optimizations and relative binding energy calculations
indicated that the [NiCl2(P,P)] complexes have enhanced
thermodynamic stability in comparison to the [NiCl2(P,PS)]
and [NiCl2(PS,PS)] complexes, and this is due to the
stronger covalent character of the Ni−P vs the Ni−S bond. The
series of NiCl2 complexes (2, 3 and 5, 6) of N-thioether-
Within the P,P derivatives, we found better activities for the
N-aryl complex 5 than for the N-alkyl complex 2 at lower Al/Ni
ratios, with TOFs of 27100 (3 equiv of AlEtCl2) and 48400 (10
equiv of AlEtCl2) mol of C2H4/((mol of Ni) h) (entries 11 and
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3+5
129.52 (s, Ar, Cpara), 128.46 (virtual t,
J
= 6.5 Hz, Ar, Cmeta),
functionalized DPPA-type ligands (1, 1·S and 4, 4·S) was
applied to the catalytic oligomerization of ethylene. Using
AlEtCl2 as cocatalyst, mostly ethylene dimers were produced,
with significant amounts of trimers (selectivity in the range 11−
36%). Productivities up to 40400 and 48200 g of C2H4/((g of
Ni) h), with corresponding TOF values of 84800 and 101100
mol of C2H4/((mol of Ni) h), were obtained with precatalysts
2 and 3, respectively. It was clearly shown that both factors, (i)
the chelate ring size formed upon coordination of the P,P and
the P,PS ligands to the Ni(II) center and (ii) the nature of
the N-substituent, by comparison of the N-alkyl thioether and
N-aryl thioether species, are important molecular character-
istics, at constant cocatalyst, that should be considered in the
development of active and selective (pre)catalysts for ethylene
oligomerization. However, there is currently no clear
explanation for the different catalytic performances as a
function of the P,P or P,S donors and associated change in
chelate ring size, of the N-substituent, and of the excess of
cocatalyst used. It would be interesting in future work to
examine to what extent interactions between the S atom of the
N-substituent and the cocatalyst has an impact on its properties
and also, perhaps, by bringing it closer to the nickel center.
C,P
126.77 (s, N(C6H4)S, Cmeta/N), 16.34 (s, SCH3) ppm. 31P{1H} NMR
(CDCl3, 121.5 MHz): δ 69.36 (s) ppm.
Ligand 4·S2. To a solution of 4 (0.300 g, 0.59 mmol) in 15 mL of
toluene was added elemental sulfur (0.038 g, 1.18 mmol). The
solution was heated to 115 °C for 3 h. After the mixture was cooled to
room temperature, pentane was added to the solution and a
microcrystalline white powder of 4·S2 precipitated. Colorless crystals
suitable for single-crystal X-ray diffraction were grown from a toluene/
pentane mixture. Yield: 0.300 g, 89%. Anal. Calcd for C31H27NP2S3
(571.70): C, 65.13; H, 4.76; N, 2.45. Found: C, 65.18; H, 4.90; N,
2.36. FTIR: νmax(solid)/cm−1 3053 w, 2923 vw, 2858 vw, 2827 vw,
2682 vw, 2581 vw, 2364 w, 2344 w, 2324 w, 1586 w, 1560 w, 1491 m,
1480 m, 1437 s, 1405 w, 1308 mw, 1197 m, 1178 m, 1094 s, 1018 w,
999 w, 968 w, 953 s, 916 s, 880 s, 816 m, 746 w, 732 w, 715 vs, 697 vs,
1
686 vs, 645 vs, 631 w, 613 m. H NMR (CDCl3, 300 MHz): δ 8.07−
8.03 (m, 8H, Ar), 7.54 (AA′ part of a AA′BB′ spin system, 2H, 3JH,H
=
8.4 Hz, N(C6H4)S, Hmeta/N), 7.28−7.25 (m, 4H, Ar), 7.20−7.17 (m,
3
8H, Ar), 6.71 (BB′ part of a AA′BB′ spin system, 2H, JH,H = 8.4 Hz,
N(C6H4)S, Hortho/N), 2.23 (s, 3H, SCH3) ppm. 13C{1H} NMR
2
(CDCl3, 75.5 MHz): δ 137.59 (t, JC,P = 2.0 Hz, N(C6H4)S, Cipso/N),
135.83 (s, N(C6H4)S, Cipso/S), 133.61−133.48 (five-line multiplet of a
2+4
simulated AXX′ spin system,
J
= 11.1 Hz (4JAX′ ≈ 0 Hz), Ar,
C,P
Cortho), 132.73 (t, 3JC,P = 3.0 Hz, N(C6H4)S, Cortho/N), 131.61 (d, 1JC,P
4+6
= 101.5 Hz, Ar, Cipso), 131.46 (virtual t,
J
= 1.2 Hz, Ar, Cpara),
C,P
127.69−127.53 (five line multiplet of a simulated AXX′ spin system,
3+5
4
= 13.9 Hz (5JAX′ ≈ 0 Hz), Ar, Cmeta), 125.59 (t, JC,P = 1.5 Hz,
EXPERIMENTAL SECTION
J
■
C,P
N(C6H4)S, Cmeta/N), 15.54 (s, SCH3) ppm. 31P{1H} NMR (CDCl3,
121.5 MHz): δ 69.03 (s) ppm.
General Procedures. All operations were carried out using
standard Schlenk techniques under an inert atmosphere. Solvents were
purified and dried under nitrogen by conventional methods. CD2Cl2
and CDCl3 were dried over 4 Å molecular sieves, degassed by freeze−
pump−thaw cycles, and stored under argon. NMR spectra were
recorded at room temperature on a Bruker AVANCE 300
spectrometer (1H, 300 MHz; 13C, 75.47 MHz; 31P, 121.49 MHz)
and referenced using the residual proton solvent (1H) or solvent (13C)
resonance. Chemical shifts (δ) are given in ppm. IR spectra were
recorded in the region 4000−100 cm−1 on a Nicolet 6700 FT-IR
spectrometer (ATR mode, SMART ORBIT accessory, Diamond
crystal). Elemental analyses were performed by the “Service de
Ligand 4·S. To a solution of 4 (0.220 g, 0.43 mmol) in 15 mL of
CH2Cl2 was added elemental sulfur (0.021 g, 0.65 mmol). The
solution was stirred at room temperature for 30 min. Evaporation of
the volatiles led to the formation of a pale yellow oil that was dissolved
in 10 mL of toluene. Addition of pentane to this solution led to the
precipitation of a white crystalline powder of 4·S2. After filtration and
evaporation of the volatiles, a colorless oil was obtained. Its
examination by 31P{1H} NMR spectroscopy revealed the presence of
4·S (ca. 70%) and 4·S2 (ca. 30%). We could not isolate pure 4·S, and
in the subsequent complexation reactions, 4·S was thus prepared in
situ. Purification of the complexes with 4·S and 4·S2 was readily
achieved (see below). 1H NMR (CD2Cl2, 300 MHz): δ 8.11−8.05 (m,
4H, Ar), 8.02−7.05 (m, 4H, Ar), 7.43−7.31 (m, 12H, Ar), 6.68 (AA′
́
microanalyses”, Universite de Strasbourg. Electrospray mass spectra
(ESI-MS) were recorded on a microTOF (Bruker Daltonics, Bremen,
Germany) instrument using nitrogen as drying agent and nebulizing
gas. Gas chromatographic analyses were performed on a Thermoquest
GC8000 Top series gas chromatograph using a HP Pona column (50
m, 0.2 mm diameter, 0.5 μm film thickness). The complexes
[NiCl2(DME)],33 [Ni(NCMe)6](BF4)2,34 [PdCl2(COD)],35 2,15 and
315 were prepared according to literature methods. All other reagents
were used as received from commercial suppliers. All iteration-fitting
calculations and simulations of the 31P{1H} NMR spectra were
performed by using the Bruker WIN-DAISY program.36
3
part of a AA′BB′ spin system, d, 2H, JH,H = 8.6 Hz, N(C6H4)S,
3
Hmeta/N), 6.59 (BB′ part of a AA′BB′ spin system, d, 2H, JH,H = 8.6
Hz, N(C6H4)S, Hortho/N), 2.27 (s, 3H, S-CH3) ppm. 31P{1H} NMR
(CDCl3, 121.5 MHz): δ 72.38 (d, 2JP,P = 104.5 Hz, PV), 55.34 (d, 2JP,P
= 104.5 Hz, PIII) ppm.
Synthesis of the Ni(II) Complexes. Complex 5. To a suspension
of [NiCl2(DME)] (0.130 g, 0.59 mmol) in CH2Cl2 (10 mL) was
added a solution of 4 (0.300 g, 0.59 mmol) in CH2Cl2 (20 mL). The
solution quickly turned orange-red and was stirred at room
temperature for 4 h. The solvent was removed under reduced
pressure, and the red solid obtained was washed with pentane. Red
crystals suitable for single-crystal X-ray diffraction were grown from a
CH2Cl2/pentane mixture. Yield: 0.312 g, 83%. Anal. Calcd for
C31H27Cl2NNiP2S (637.16): C, 58.44; H, 4.27; N, 2.20. Found: C,
58.48; H, 4.19; N, 2.31. FTIR: νmax(solid)/cm−1 3053 w, 2919 w, 1589
m, 1492 ms, 1480 m, 1433s, 1332 vw, 1309 w, 1255 vs, 1182 w, 1162
vw, 1105 vs, 1096 vs, 1023 vw, 1007 vw, 997 w, 951 s, 907 vs, 852 vw,
824 m, 749 w, 736 s, 720 w, 694 vw, 686 vs, 562 w, 548 s, 508 m, 495
m, 483 vs, 448 vw, 439 vw, 415 w, 343 w, 326 vs, 269 w, 236 w, 214 w,
172 vw, 157 vw, 142 w. 1H NMR (CD2Cl2, 300 MHz): δ 8.01 (q, 8H,
Synthesis of the Ligands. Ligand 4. An excess of chlorodiphe-
nylphosphine (6.342 g, 28.73 mmol) was added dropwise to a solution
of 4-(methylthio)aniline (2.000 g, 14.37 mmol) and triethylamine
(3.631 g, 35.91 mmol) in 150 mL of THF under an inert atmosphere.
The reaction mixture was stirred for 3 h. After filtration, the volatiles
were removed under reduced pressure and the residue was washed
twice with pentane to give 4 as a colorless solid. Yield: 5.178 g, 71%.
Anal. Calcd for C31H27NP2S (507.57): C, 73.36; H, 5.36; N, 2.76.
Found: C, 72.98; H, 5.48; N, 2.51. FTIR: νmax(solid)/cm−1 3050 w,
3022 vw, 2983 vw, 2922 vw, 1489 m, 1477 m, 1432 s, 1398 w, 1327
vw, 1308 w, 1266 vw, 1217 m, 1174 m, 1110 w, 1093 ms, 1070 w,
1024 w, 1012 w, 996 w, 974 w, 954 w, 941 w, 916 w, 886 vs, 807 m,
741 vs, 707 vw, 691 vs. 1H NMR (CD2Cl2, 300 MHz): δ 7.40−7.27 (
3JH,H = 7.0 Hz, Hmeta of Ar), 7.71 (t, 4H, JH,H = 7.0 Hz, Hpara of Ar),
3
3
3
m, 20H, Ar), 6.84 (AA′ part of a AA′BB′ spin system, d, 2H, JH,H
=
7.57 (t, 8H, JH,H = 7.0 Hz, Hortho of Ar), 6.84 (AA′ part of a AA′BB′
spin system, 2H, 3JH,H = 8.4 Hz, N(C6H4)S, Hmeta/N), 6.38 (BB′ part of
8.6 Hz, N(C6H4)S, Hmeta/N), 6.56 (BB′ part of a AA′BB′ spin system
3
3
d, 2H, JH,H = 8.6 Hz, N(C6H4)S, Hortho/N), 2.35 (s, 3H, SCH3) ppm.
a AA′BB′ spin system, 2H, JH,H = 8.4 Hz, N(C6H4)S, Hortho/N), 2.32
13C{1H} NMR (CD2Cl2, 75.5 MHz): δ 145.00 (t, JC,P = 2.1 Hz,
2
(s, 3H, SCH3) ppm. 13C{1H} NMR (CD2Cl2, 75.5 MHz): δ 137.59 (s,
1+3
2
N(C6H4)S, Cipso/N), 139.54 (virtual t,
J
= 13.6 Hz, Ar, Cipso),
N(C6H4)S, Cipso/S), 136.17 (t, JC,P = 5.6 Hz N(C6H4)S, Cipso/N),
C,P
2+4
135.50 (t, 5JC,P = 0.9 Hz, N(C6H4)S, Cipso/S), 133.65 (virtual t, 2+4JC,P
=
133.66 (virtual t,
J
= 11.2 Hz, Cortho, Ar), 133.24 (virtual t, 4+6JC,P
C,P
3
3+5
23.1 Hz, Ar, Cortho), 129.94 (t, JC,P = 3.0 Hz, N(C6H4)S, Cortho/N),
= 1.5 Hz, Cpara, Ar), 129.28 (virtual t,
J
= 11.9 Hz, Cmeta, Ar),
C,P
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Organometallics
Article
1+3
126.80 (virtual t,
J
= 50.8 Hz, Cipso, Ar), 126.63 (s, N(C6H4)S,
magnetically stirred (900 rpm) 145 mL stainless steel autoclave. A
125 mL glass container was used to protect the inner walls of the
autoclave from corrosion. All catalytic tests were started at 30 °C, and
no cooling of the reactor was done during the reaction. After injection
of the catalytic solution and of the cocatalyst under a constant low flow
of ethylene, the reactor was pressurized to 10 bar. A temperature
increase was observed which resulted solely from the exothermicity of
the reaction. The 10 bar working pressure was maintained during the
experiments through a continuous feed of ethylene from a reserve
bottle placed on a balance to allow continuous monitoring of the
ethylene uptake. The oligomerization products and remaining ethylene
were only collected from the reactor at the end of the catalytic
experiment. At the end of each test (35 min) a dry ice bath, and in the
more exothermic cases also liquid N2, was used to rapidly cool the
reactor, thus stopping the reaction. When the inner temperature
reached 0 °C, the ice bath was removed, allowing the temperature to
slowly rise to 10 °C. The gaseous phase was then transferred into a 10
L polyethylene tank filled with water. An aliquot of this gaseous phase
was transferred into a Schlenk flask, previously evacuated, for GC
analysis. The products in the reactor were hydrolyzed in situ by the
addition of ethanol (1 mL), transferred into a Schlenk flask, and
separated from the metal complexes by trap-to-trap evaporation (20
°C, 0.3 mmHg) into a second Schlenk flask previously immersed in
liquid nitrogen in order to avoid any loss of product. For GC analyses,
1-heptene was used as an internal reference. AlEtCl2 was used as a
cocatalyst and, depending on the amount used (3, 10, or 40 equiv), the
required quantity of Ni(II) complex was dissolved in 13.5 (3 equiv of
EADC) or 10 mL (10 and 40 equiv of EADC) of chlorobenzene, and
this solution was injected into the reactor. The cocatalyst solution, 1.5
or 5 mL corresponding to 3 or 10 and 40 equiv, respectively, was then
added. Therefore, the total volume of the solution inside the reactor
was 15 mL.
C,P
3
Cmeta/N), 124.55 (t, JC,P = 3.4 Hz, N(C6H4)S, Cortho/N), 15.15 (s,
SCH3) ppm. 31P{1H} NMR (CD2Cl2, 121.5 MHz): δ 45.38 (s) ppm.
MS (ESI): m/z 600.00 [M − Cl]+.
Complex 6. To a solution of 4 (0.300 g, 0.59 mmol) in CH2Cl2 (20
mL) was added elemental sulfur (0.028 g, 0.88 mmol). The solution
was stirred at room temperature for 30 min and then added to a
suspension of [NiCl2(DME)] (0.065 g, 0.29 mmol) in CH2Cl2 (20
mL). The mixture quickly turned bright purple and was stirred at
room temperature for 4 h. After filtration, the solvent was removed
under reduced pressure and the bright purple solid was washed with
toluene. Purple single crystals suitable for X-ray diffraction were grown
by slow evaporation of an acetone/pentane mixture. Yield (based on 4·
S formed in situ and estimated by 31P NMR): 0.132 g, 82%. Anal.
Calcd for C28H29Cl2NNiP2S2·(acetone) (727.31): C, 55.31; H, 4.65;
N, 1.95. Found: C, 54.96; H, 4.54; N, 1.88. FTIR: νmax(solid)/cm−1
3057 w, 2921 w, 2682 w, 2584 w, 1706 s, 1583 w, 1560 w, 1483 s,
1432 vs, 1400 vw, 1359 m, 1309 w, 1222 w, 1201 s, 1179 m, 1161 w,
1098 vs, 1010 w, 998 w, 973 s, 937 m, 918 w, 897 vs, 745 m, 731 w,
709 w, 688 vs, 646 m, 557w , 524 s, 513 vs, 489 w, 480 w, 455 vw, 440
w, 415 vw, 393 vw, 382 m, 339 s, 315 w, 309 m, 291 m, 271 w, 233 w,
210 vw, 179 m, 150 s, 139 vw, 134 vw, 121 vw, 108 w. 31P{1H} NMR
(CDCl3, 121.5 MHz): δ 83.81 (br s, see text). MS (ESI): m/z 632.01
[M − Cl]+, 1298.99 [Ni(4·S)2][NiCl3]+.
Complex 7. To a solution of 4 (0.200 g, 0.40 mmol) in CH2Cl2 (20
mL) was added elemental sulfur (0.019 g, 0.60 mmol). The solution
was stirred at room temperature for 30 min and then added to a
suspension of [Ni(NCMe)6](BF4)2 (0.039 g, 0.10 mmol) in CH2Cl2
(20 mL). The mixture quickly turned yellow-orange, and stirring at
room temperature was maintained for 4 h. After filtration, the solvent
was removed under reduced pressure and the orange solid was washed
with toluene. Red crystals suitable for single-crystal X-ray diffraction
were grown from a CH2Cl2/pentane mixture. Yield (based on 4·S
formed in situ and estimated by 31P NMR): 0.105 g, 85%. Anal. Calcd
for C62H54N2NiP4S4BF4 (1131.56): C, 56.78; H, 4.15; N, 2.13. Found:
C, 56.64; H, 4.24; N, 2.00. FTIR: νmax(solid)/cm−1 3048 vw, 2920 vw,
2679 vw, 1583 w, 1480 m, 1437 s, 1400 vw, 1318 w, 1286 vw, 1202 m,
1095 m, 1048 vs, 1037 w, 1024 vw, 996 w, 960 w, 933 w, 925 w, 890
m, 812 vw, 756 w, 744 w, 733 m, 705 w, 689 s, 634 w, 620 m, 598 s,
558 w, 521 s, 509 s, 493 s, 458 m, 432 w, 395 w, 382 m, 364 m, 353
vw, 320 w, 255 w, 235 w, 221 w, 194 m, 179 m, 140 vw, 125 w, 117
vw, 106 vs. 1H NMR (CDCl3, 300 MHz): δ 7.72−7.56 (m, 40H, Ar),
ASSOCIATED CONTENT
■
S
* Supporting Information
Tables giving X-ray data collection, structure solution, and
structure refinement details for compounds 4, 4·S2, 5·2CH2Cl2,
and 6·C3H6O (Table S1) and for compounds 7·4CH2Cl2, 8·
CH2Cl2 and 9 (Table S2) and CIF files giving crystallographic
data for all seven compounds, and text, tables, and figures
giving structural comparisons between ligand 4 and three p-
phenyl-functionalized DPPA-type ligands, the 31P{1H} NMR
spectra of complex 7 and their evolution after addition of NEt3·
HCl (1 and 2 equiv), and computational details. This material
is available free of charge via the Internet at http://pubs.acs.org.
CIF files of these compounds have also been deposited with the
CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K., and can be
obtained on request free of charge, by quoting the publication
citation and deposition numbers 873232 and 985336−985341.
3
6.64 (AA′ part of a AA′BB′ spin system, 4H, JH,H = 8.6 Hz,
N(C6H4)S, Hmeta/N), 6.06 (BB′ part of a AA′BB′ spin system, 4H, 3JH,H
= 8.6 Hz, N(C6H4)S, Hortho/N), 2.26 (s, 6H, SCH3) ppm. 13C{1H}
NMR (CDCl3, 75.5 MHz): δ 142.10 (s, N(C6H4)S, Cipso/S), 135.64 (d,
4JC,P = 2.6 Hz, Cpara, Ar−PV), 134.29 (s, Cpara, Ar−PIII), 134.00 (virtual
2+4+5
2
t,
J
= 13.0 Hz, Cortho of Ar−PIII), 133.03 (d, JC,P = 11.6 Hz,
C,P
Cortho, Ar−PV), 131.53 (s, N(C6H4)S, Cortho/N), 130.31−130.20 (m,
Cmeta, Ar−PIII + Ar−PV), 129.96 (t, JC,P = 2.1 Hz, Cipso/N), 125.86 (s,
2
1+3+4
N(C6H4)S, Cmeta/N), 124.53 (virtual t,
J
= 56.7 Hz, Cipso, Ar−
C,P
PIII), 125.35 (d, JC,P = 96.3 Hz, Cipso, Ar−PV), 14.73 (s, SCH3) ppm.
31P{1H} NMR (CD2Cl2, 121.5 MHz): δ two triplets of a AA′BB′ spin
system (Scheme 5): δA 105.80 ppm, δB 84.61 ppm; spectroscopic
simulations afforded the coupling constants 2+3J(PAPB) = 2+3J(PA′PB′)
1
AUTHOR INFORMATION
■
3
4
2
Corresponding Authors
= 77.30 Hz, J(PAPB′) = 66.34 Hz, J(PAPA′) = 76.44 Hz, J(PBPB′) =
76.39 Hz. MS (ESI): m/z 568.07 [M]2+.
*C.F.: e-mail, christophe.fliedel@fct.unl.pt; fax, +351 212 948
550.
*P.B.: e-mail, braunstein@unistra.fr; fax, +33 368 851 322.
General Procedure for the Catalytic Ethylene Oligomeriza-
tion Reaction. All catalytic reactions were carried out in a
Present Addresses
Scheme 5. Spin System Used for the Assignment of the
31P{1H} NMR Resonances of Complex 7
†REQUIMTE, Departamento de Quimica, Faculdade de
̂
Ciencias e Tecnologia, Universidade Nova de Lisboa, Caparica
2829-516, Portugal.
‡Institut fur Anorganische Chemie, RWTH Aachen University,
̈
Landoltweg 1, 52074 Aachen, Germany
Notes
The authors declare no competing financial interest.
2532
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Organometallics
Article
Chemicals Ltd) WO01/10876, 2001. (g) Hofmann, P.; Perez-Moya, L.
ACKNOWLEDGMENTS
■
A.; Krause, M. E.; Kumberger, O.; Muller, G. Z. Naturforsch., B 1990,
̈
We are grateful to the CNRS, the Minister
(Paris), the DFH/UFA (International Research Training
Group 532-GRK532, Ph.D. grants to C.F. and A.G.), and the
̀
e de la Recherche
45b, 897.
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̂
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SFRH/BPD/73253/2010 to C.F. and SFRH/BPD/44262/
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́
Pierre de Fremont and the Service de Radiocrystallographie,
Institut de Chimie (UMR 7177 CNRS-UdS), for performing
the X-ray diffraction studies, the UdS High-Performance
Computing Center and the Laboratoire de Chimie Quantique
(UdS) for the provision of computational facilities, and Marc
Kowaldt, F. H.; Kurschner, U.; Limbacker, B.; Sistig, F. P.
̈
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Mermillon-Fournier and Melanie Boucher for providing
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