Phosphino-indenyl complexes of nickel(II)

Article abstract of DOI:10.1139/v05-079

The BH₃-protected phosphinoindenyl ligand indenyl(CH ₂)₂PPh₂·BH₃ was used in the preparation of (η^5/3:η⁰-indenyl(CH ₂)₂PPh₂·BH₃)Ni(PPh ₃)Cl, which has been characterized by NMR spectroscopy and X-ray diffraction studies. On the other hand, all attempts at preparing the closely related complex (η^5/3:η¹indenyl(CH ₂)₂PPh₂)NiCl, in which the tethered phosphine moiety is coordinated to the Ni centre, were unsuccessful. One of these unsuccessful attempts yielded instead the novel indenyl-PCP pincer complex {K^P,K^C,K^P-1,3-(CH₂CH ₂PPh₂)₂-2-indenyl}NiCl, which has been characterized by X-ray diffraction studies.

Full text of DOI:10.1139/v05-079

634
Phosphino-indenyl complexes of nickel(II)1
Laurent F. Groux, Francine Bélanger-Gariépy, and Davit Zargarian
Abstract: The BH3-protected phosphinoindenyl ligand indenyl(CH2)2PPh2·BH3 was used in the preparation of (η5/3:η0-
indenyl(CH2)2PPh2·BH3)Ni(PPh3)Cl, which has been characterized by NMR spectroscopy and X-ray diffraction studies.
On the other hand, all attempts at preparing the closely related complex (η5/3:η1-indenyl(CH2)2PPh2)NiCl, in which the
tethered phosphine moiety is coordinated to the Ni centre, were unsuccessful. One of these unsuccessful attempts
yielded instead the novel indenyl-PCP pincer complex {κP,κC,κP-1,3-(CH2CH2PPh2)2-2-indenyl}NiCl, which has been
characterized by X-ray diffraction studies.
Key words: indenyl complexes, tethered phosphines, PCP pincer complexes.
Résumé : Le ligand protégé indenyl(CH2)2PPh2·BH3 a permis la préparation du composé (η5/3:η0-indenyl(CH2)2-
PPh2·BH3)Ni(PPh3)Cl. Ce dernier a été complètement caractérisé par la spectroscopie RMN et études cristallographiques.
Toutes les tentatives de préparer et isoler le complexe (η5/3:η1-indenyl(CH2)2PPh2)NiCl ont échoué. En revanche, nous
avons obtenu lors d’une de ces tentatives un nouveau complexe inattendu; une étude cristallographique a démontré que
ce dernier était un complexe de type « pincer » PCP-indényle, soit {κP,κC,κP-1,3-(CH2CH2PPh2)2-2-indenyl}NiCl.
Mots clés : complexes d’indényle, phosphines attachées, complexe de type « pincer » PCP.
Groux et al. 639
Introduction
known only for Cr (7), Ru (8), Rh (9), and Ir (9b). There-
fore, we set out to prepare (Ind^PR2)NiX and probe the in-
fluence of the chelating PR2 moiety on reactivities. The
present report describes our attempts to prepare the complex
(η5/3:η1-IndCH2CH2PPh2)NiCl.
Previous studies have shown that the complexes
(Ind)Ni(PR3)X (Ind = indenyl and its substituted derivatives;
R = Ph, Me, Cy; X = Cl, Me, CC-Ph, etc.) (1) are active
precatalysts for the oligo- and polymerization of alkenes (2),
alkynes (3), and PhSiH3 (4) and the hydrosilylation of
olefins and ketones (5). The available mechanistic evidence
indicates that the steric and (or) electronic properties of the
phosphine ligand and its relative lability influence the rates
of these reactions and the catalytic activities. Thus, the PMe3
derivatives are much more active than their PPh3 analogues
in the polymerization of ethylene (2a) and PhSiH3 (4),
whereas the opposite order of reactivity is found in the
hydrosilylation reactions (5).
To examine further the influence of phosphines on the
above reactions, we considered studying a series of com-
plexes featuring ligands of the type Ind^PR2, with ^ repre-
senting a side chain connecting the PR2 and Ind moieties.
Our reasoning was that since the chelating phosphine moiety
in (Ind^PR2)NiX should be less prone to dissociation, study-
ing the reactivities of such complexes should allow a direct
measure of the impact of phosphine lability on the reactivi-
ties of this family of complexes. A search of the literature
showed that complexes bearing the analogous Cp^PR2 lig-
ands are known for many transition metals (except those of
group 10) (6), whereas complexes of Ind^PR2 ligands are
Results and discussion
The ligand Ind(CH2)2PPh2 (1) was prepared by slightly
modified versions of previously reported procedures (10,
11), consisting of (a) successive nucleophilic displacements
of Cl– in ClCH2CH2Cl by [PPh2]– and [Ind]– or (b) the
reductive cleavage of spiro(cyclopropane-1,1′-indene) (12)
by [PPh2]– (Scheme 1). Crude samples of 1 obtained from
these routes were reacted with BH3·THF to protect the
phosphine moiety from oxidation; pure samples of
Ind(CH2)2PPh2·BH3 (1·BH3) were then isolated by flash
chromatography and completely characterized by NMR and
X-ray diffraction studies (13). Treatment of 1·BH3 with H+,
followed by neutralization with Na2CO3, removed the pro-
tecting group and gave pure samples of 1, which were used
in subsequent experiments. Alternatively, crude samples of 1
were used in a few instances for the preparation of the target
complex; for this purpose, route (b) proved more convenient
because the in situ generated anion [Ind(CH2)2PPh2]– obvi-
ated the need for an additional deprotonation step.
Received 1 December 2004. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 9 June 2005.
This contribution is dedicated to Professor H. Alper in recognition of his seminal contributions to organometallic catalysis.
L.F. Groux, F. Bélanger-Gariépy, and D. Zargarian.2 Département de chimie, Université de Montréal, Montréal, QC H3C 3J7,
Canada.
1This article is part of a Special Issue dedicated to Professor Howard Alper.
2Corresponding author (e-mail: zargarian.davit@umontreal.ca).
Can. J. Chem. 83: 634–639 (2005)
doi: 10.1139/V05-079
© 2005 NRC Canada

Groux et al.
635
Scheme 1.
IndLi / THF
Ind
LiPPh2
THF
Cl
PPh2
H2O
a)
Cl
Cl
BH3·THF
.
PPh2 BH3
PPh2
Ind
1. H+
2. Na2CO3
.
1 BH3
1
LiPPh2
THF
Li[IndCH2CH2PPh2]
b)
Scheme 2.
[R-Ind]-
R
- PPh3
(PPh3)2NiCl2
(PPh3)2NiCl(η1-R-Ind)
Ni
[R-Ind]-
Cl
Ph3P
(PPh3)2Ni(η1-R-Ind)2
R
(PPh3)2NiCl2
(PPh3)nNi(0)
(PPh3)3NiCl
R
The main synthetic route used for the attempted synthesis
of the target complex was based on the procedure used for
the preparation of IndNi(PR3)Cl (14). Thus, ligand 1 was
deprotonated with BuLi and added to an Et2O suspension of
Ni(PPh3)2Cl2. This addition changed the forest-green colour
of the initial mixture to dark red, the typical colour of
IndNi(PR3)Cl; shortly thereafter, however, the reaction mix-
ture turned beige-brown and a beige solid precipitated, im-
plying the onset of a decomposition process. The 31P{1H}
NMR spectrum of the filtrate obtained from filtration of the
final mixture showed that the reaction had produced a num-
ber of phosphine-containing species,3 while the observation
lowed the successful isolation of (η5/3:η1-IndCH2CH2-
PPh2)NiCl. The precise reasons for the failure to obtain our
target complex are not known; we speculate, however, that
the dark-red species formed during the initial stages of these
reactions is an intermediate species such as (η5/3:η0-Ind-
CH2CH2PPh2)Ni(PPh3)Cl, 2. The apparent instability of this
species is in contrast to the stability of the analogous com-
plexes (η5/3:η0-Ind^NR2)Ni(PPh3)Cl, which have been syn-
thesized successfully (2c, 17). The discrepancy in the
stabilities of these complexes leads us to suspect that the pit-
fall of our synthetic approach lies with the incompatible
intramolecular interaction of the chelating phosphine moiety
with the Ni centre.
1
of several broad signals in the H NMR spectrum indicated
that one or more paramagnetic species had also formed. The
1H NMR spectrum of the beige solid obtained from the fil-
tration also showed broad and featureless signals. Cooling
the beige filtrate yielded yellow crystals that were identified
as the Ni(I) complex (PPh3)3NiCl (15). The formation of this
species has been observed in previous studies (16) and sig-
nals a side reaction involving the nickel-catalyzed coupling
of two indenyl ligands (Scheme 2).
The above hypothesis was borne out by the finding that
the above-proposed intermediate is indefinitely stable when
the PPh2 moiety is coordinated by BH3. Thus, using the
BH3-protected ligand 1·BH3 in our syntheses allowed the
successful isolation of the complex (η3/5:η0-IndCH2CH2-
PPh2·BH3)Ni(PPh3)Cl, 2·BH3 (Scheme 3). The structure pro-
posed for this complex was corroborated by the NMR
spectra, as follows. For instance, the 31P{1H} NMR spec-
trum of 2·BH3 showed a sharp singlet at 29.9 ppm (parts per
million), attributed to the Ni-PPh3 moiety; for comparison,
the corresponding signal in (Ind-CH2CH2NMe2)Ni(PPh3)Cl
The above procedure was repeated several times under
different reaction conditions (e.g., solvent, temperature, order
of mixing, source of nickel, etc.), but no set of conditions al-
3 The 31P{1H} NMR spectrum (C6D6) displayed the following signals: 44.6, 33.6, 33.3, 32.8, 32.5, 30.3, 30.1, 29.9, 29.4, 29.1, 24.4
(O=PPh3), 20.7, 3.2, –4.9 (free PPh3). The signals displayed in the corresponding spectrum in CDCl3 were as follows: 58.7, 57.9, 55.9, 34.2,
33.6, 32.5, 30.2, 29.5 (O=PPh3), 24.8, 23.3, 21.6, 20.2, –3.5, –4.8 (free PPh3), –14.0, –15.0.
© 2005 NRC Canada

636
Can. J. Chem. Vol. 83, 2005
Scheme 3.
BH3
Ph2P
1. BuLi
.
1 BH3
.
+ PPh3 BH3
Ni
.
Ni
X
2. Ni(PPh3)2Cl2
Cl
Cl
P
Ph2
PPh3
2 BH3
Scheme 4.
PPh2
PPh2
PPh2
1. BuLi
NiCl2
1
Cl
Ni
2. ClCH2CH2PPh2
∗
∗
PPh2
indenyl ligand perpendicular to the coordination plane. The
tethered Ph2P·BH3 moiety points away from the metal cen-
tre, and the absence of any P–Ni interaction results in a vir-
tually unchanged P—BH3 distance (1.912(3) Å in 2·BH3 vs.
1.923(3) Å in 1·BH3). The Ind hapticity is distorted toward
an unsymmetrical, trihapto mode, as reflected in the follow-
ing structural parameters: (i) the Ni—C3a and Ni—C7a
bond distances (avg. 2.34 Å) are much longer than the Ni—
C1 and Ni—C3 distances (avg. 2.08 Å); (ii) the Ni—C1 dis-
tance (ca. 2.14 Å) is significantly longer than the Ni—C3
distance (ca. 2.03 Å); (iii) the slip parameters4 ∆(M—C)
(0.26 Å), HA (ca. 8.7°), and FA (ca. 8.3°) are in the ex-
pected range for η5/η3-Ind ligands (18). This type of “slip-
page” is attributed to (a) the tendency of Ni(II) to avoid an
18-electron configuration and (b) the different trans influ-
ences of PPh3 and Cl ligands, respectively (1).
Fig. 1. ORTEP plot of 2·BH3. Hydrogen atoms have been omit-
ted for clarity. Selected distances (Å) and angles (°): Ni—Cl
2.143(7), Ni—C2 2.054(6), Ni—C3 2.031(7), Ni—C3A 2.321(7),
Ni—C7A 2.365(8), Ni—P1 2.179(3), Ni—Cl 2.170(2), C1—C2
1.435(8), C2—C3 1.419(8), C3—C3A 1.397(9), C3A—C7A
1.439(9), C7A—C1 1.462(8), P2—B 1.912(8); C1-Ni-P1
164.0(2), C3-Ni-C1 160.0(2), P1-Ni-C1 101.09(9), C1-Ni-C1
94.7(2), P1-Ni-C3 98.0(2), C1-Ni-C3 66.0(3).
With complex 2·BH3 at hand, we attempted to prepare 2
by thermally inducing the transfer of BH3 from the tethered
PPh2 moiety to the PPh3 ligand and the subsequent elimina-
tion of Ph3P·BH3; unfortunately, however, no BH3-exchange
was observed between the phosphine moieties. On the other
hand, direct deprotection of the tethered PPh2 moiety by
amines led instead to decomposition. It appears, therefore,
that the target complex (η5/3:η1-IndCH2CH2PPh2)NiCl is in-
herently unstable. Although modeling studies do not support
the hypothesis that this instability is caused by the CH2CH2
tether, future attempts will investigate the feasibility of pre-
paring analogous complexes bearing longer side chains.
resonates at 30.8 ppm (see ref. 17a). In addition, we ob-
served a peak at 17.5 ppm that was broadened as a result of
coupling to the quadrupolar 11B nucleus (cf. 1·BH3:
1
A new PCP–Ni complex
16.6 ppm). The H NMR spectrum contained a broad singlet
at ca. 1.2 ppm assigned to BH3, in addition to the character-
istic signals for the Ind protons (e.g., a signal at ca. 3.5 ppm
assigned to H3). The unequivocal confirmation of the pro-
posed structure was furnished by the results of X-ray dif-
fraction studies, described below.
Complex 2·BH3 adopts virtually the same structure as
many of the previously reported Ni-indenyl complexes (1).
The coordination geometry around Ni (Fig. 1) can be de-
scribed as a distorted square-planar geometry with the
One of the attempts to synthesize the target complex re-
sulted in the unexpected formation of a Ni complex bearing
the new ligand Ph2P^Ind^PPh2, shown in Scheme 4. Thus,
reacting NiCl2 with a crude batch of 1, prepared according
to route (a) in Scheme 1, gave a red mixture that yielded a
small quantity of red crystals identified by X-ray diffraction
studies as the novel pincer complex (PCP-Ind((CH2)2-
PPh2)2)NiCl, 3. The 31P{1H} NMR spectrum of 3 showed
two AB doublets displaying large coupling constants (2JP-P
=
4 These slip parameters are defined as follows: ∆M—C = 1/2[(M—C3a + M—C7a) – (M—C1 + M—C3)]; HA is the angle between the
planes formed by the atoms C(1), C(2), C(3) and C(1), C(3), C(3a), C(7a); FA is the angle between the planes formed by the atoms C(1),
C(2), C(3) and C(3a), C(4), C(5), C(6), C(7), and C(7a).
© 2005 NRC Canada

Groux et al.
637
Fig. 2. ORTEP plot of 3. Hydrogen atoms and disordered solvent
have been omitted for clarity. Selected distances (Å) and angles
(°): Ni—Cl 2.2086(12), Ni—P1 2.1772(10), Ni—P2 2.2146(9),
Ni—C2 1.888(3), C1—C2 1.525(4), C2—C3 1.360(4), C3—C3A
1.457(4), C3A—C7A 1.391(5), C1—C7A 1.522(4); C1-Ni-P1
98.32(4), C1-Ni-P2 91.95(4), C2-Ni-P1 85.08(9), C2-Ni-P2
88.96(9), C1-Ni-C2 162.46(9), P1-Ni-P2 163.64(3).
Pincer compounds have attracted much attention recently
owing to the many interesting reactions they promote and
(or) catalyze (20), including alkane dehydrogenations (21),
ketone hydrogenations (22), Heck coupling (23), olefin
aminations (24), etc. (25). Most pincer complexes have a C2
symmetric skeleton based on an ortho,ortho-disubstituted aryl
ring, a 1,5-disubstituted pentyl chain, or a 1,3-disubstituted
Cp moiety; what sets complex 3 apart is the presence of an
optically active centre at C1. Efforts are underway to de-
velop a reliable and high-yielding synthetic route for ligands
of the type R2P^Ind^PR2 and their complexes. The reactivi-
ties of these complexes will then be investigated with a view
to exploiting the optically active character of this type of
PCP complex.
Concluding remarks
The results described in this paper indicate that (η3/5:η0-
IndCH2CH2PPh2·BH3)Ni(PPh3)Cl owes its stability to the
protection of the PPh2 moiety by BH3, because all attempts
at preparing the original target complex wherein PPh2 is co-
ordinated to the Ni centre led to decomposition. We con-
clude, therefore, that tethering the phosphine moiety to the
Ind ligand has a destabilizing effect on the complexes
IndNi(PR3)Cl. In contrast, many related complexes are sta-
ble and isolable (e.g., (η5:η1-Ind^PPh2)RhI(CO) (9), (η5:η1-
Ind^PPh2)RhIII(C(=O)R)X (R = Me, Et; X = Br, I) (26), and
(η5:η1-Ind^PR2)Cr(X)Cl (X = Cl, Me) (7)), implying that
the chelation of the tethered phosphine is not inherently
destabilizing in all cases. It remains to be seen whether Ni
complexes of Ind^PR2 bearing longer tethers will be stable
to isolation.
330 Hz), indicative of inequivalent trans phosphine moieties.
The 1H NMR spectrum displayed a broad singlet at ca.
3.6 ppm, which was assigned to H1, in addition to a number
of multiplets in the aromatic and aliphatic regions; the latter
are assigned to the chemically inequivalent5 protons on the
ethyl side chains.
X-ray diffraction studies showed that the coordination ge-
ometry around the Ni centre in 3 is slightly distorted square
planar (Fig. 2), the main source of distortion arising from
inter-ligand angles of ca. 163° for the trans ligands and 98°–
85° for the cis ligands. Two trans coordination sites are oc-
cupied by the PPh2 moieties, which form unequal Ni—P
bond lengths (ca. 2.177 and 2.215 Å). The presence of an
asymmetric centre at C1 results in the formation of two
enantiomers, but the molecule crystallizes in the centro-
symmetric space group P1 because these enantiomers exist
in a racemic mixture.
The most novel feature of complex 3 is the 2-indenyl
ligand that is η1-bonded to Ni via C2 (Ni—C(sp2) = 1.89 Å).
The localized nature of bonding within the 5-membered ring
portion of the Ind ligand is evident from the much longer
bond lengths for C1—C2 (1.525(4) Å), C7a—C1
(1.522(4) Å), and C3—C3a (1.457(4) Å) compared with
C2—C3 (1.360(4) Å). We believe that 3 is the first isolated
η1-indenyl nickel complex and one of very few 2-η1-indenyl
complexes ever reported (19). The unanticipated formation
of this compound likely results from the side reactions
shown in Scheme 4: a small amount of deprotonated 1 reacts
with a second equivalent of ClCH2CH2PPh2 to give the dou-
bly functionalized indene, 1,3-(CH2CH2PPh2)2IndH; the lat-
ter then reacts with NiCl2 by C-H activation at the 2-position
to give 3.
Experimental
General considerations
All manipulations were performed under an inert atmo-
sphere of N2 using standard Schlenk techniques and a
drybox. Dry, oxygen-free solvents were employed through-
out. Ind(CH2)2PPh3 (1) has been prepared according to mod-
ified procedures (11, 12). All other reagents used in the
experiments were obtained from commercial sources and
used as received. The NMR spectra were recorded on a
Bruker AMXR400 spectrometer (1H (400 MHz), 13C{1H}
(100.56 MHz), and 31P{1H} (161.92 MHz)).
Preparation of 2·BH3
BuLi (0.47 mL of a 2.5 mol/L solution in hexane) was
added to a solution of 1·BH3 (400 mg, 1.17 mmol) in Et2O
(200 mL) and stirred for 2 h. This solution was then added
dropwise to
a suspension of Ni(PPh3)2Cl2 (1.48 g,
2.27 mmol) in Et2O (50 mL), whereupon the solution turned
dark red immediately. The mixture was evaporated, and the
residues were dissolved in CH2Cl2 (30 mL); diluting this so-
lution with hexane (120 mL) and storing the mixture at
–20 °C gave a dark red solid precipitate (450 mg, 55%
1
yield). H NMR (CDCl3): 7.9–7.2 (PPh3 and PPh2), 7.16 (t,
3JH-H = 7.4 Hz, H6), 7.09 (d, JH-H = 8.0 Hz, H7), 6.91 (t,
3
3JH-H = 7.3 Hz, H5), 6.56 (d, JH-H = 3.0 Hz, H2), 6.08 (d,
3
5 The rigid geometry imposed by the chelating PPh2 moieties and the presence of an asymmetric centre at C1 render the methylene protons
diastereotopic.
© 2005 NRC Canada

638
Can. J. Chem. Vol. 83, 2005
Table 1. Crystal data, data collection, and structure refinement of 2 and 3.
2
3
Formula
C41H38BP2ClNi
C37H33P2ClNi·CH2Cl2
Color and habit
Dimensions (mm3)
Symmetry
Space group
a (Å)
Dark red block
0.31×0.19×0.04
Triclinic
P1
8.952(5)
13.245(9)
15.670(8)
84.66(4)
74.67(4)
81.02(5)
1767.3(18)
2
Red plate
0.37×0.31×0.14
Triclinic
P1
10.741(4)
12.875(3)
14.361(6)
65.63(3)
73.92(3)
77.76(2)
1727.4(10)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Dcalcd (g cm–1)
1.311 0
1.382
Diffractometer
Temp (K)
Nonius CAD-4
293(2)
Nonius CAD-4
293(2)
λ (Cu Kα) (Å)
1.541 78
2.548
ω/2θ scan
70.00
1.540 56
4.015
ω/2θ scan
69.81
µ (mm–1)
Scan type
θmax (°)
h,k,l range
–10 ≤ h ≤ 10
–16 ≤ k ≤ 16
–19 ≤ l ≤ 19
6688
0.050 2, 0.100 9
0.860
–13 ≤ h ≤ 13
–15 ≤ k ≤ 15
–17 ≤ l ≤ 17
4815
0.043 2, 0.111 5
0.938
Reflections used (I > 2σ (I))
R[F2 > 2σ(F2)], wR(F2)
GoF
3JH-H = 7.7 Hz, H4), 3.49 (m, H3), 3.01 and 2.56 (m, CH2),
2.18 and 2.03 (m, CH2), 1.26 (br, BH3). 13C{1H} NMR
(CDCl3): 139.7 (d), 138.6 (d), 137.6 (d), 137.6, 137.1, 136.9,
136.8, 136.1, 134.6, 134.5, 134.0 (d), 132.4 and 132.3 (C5
and C6), 123.6 and 122.7 (C4 and C7), 115.0 (m, C1), 107.5
(C2), 68.2 (C3), 22.1 (d, 1JC-P = 45 Hz, CH2P), 20.0 (ind-C).
31P{1H} NMR (CDCl3): 29.9 (s, PPh3), 17.1 (br, PPh2BH3).
Anal. calcd.: C 70.59, H 5.49; found: C 70.22, H 5.55.
selected bond distances and angles are given in the figure
captions.
Acknowledgements
We gratefully acknowledge the Natural Sciences and En-
gineering Research Council of Canada (NSERC), le fond
FCAR of Quebec, and Université de Montréal for financial
support. Christophe Valla is thanked for contributing to the
preparation of complex 3.
Crystal structure determinations6
Dark red crystals of 2·BH3 and red crystals of 3 were ob-
tained from CH2Cl2–hexane solutions kept at –20 °C. The
crystal data for 2·BH3 and 3 were collected on a Nonius
CAD-4 diffractometer with graphite-monochromated Cu Kα
radiation at 223(2) K using the CAD-4 software (27). Re-
finement of the cell parameters was done with the CAD-4
software, while the data reduction used NRC-2 and NRC-2A
(28). Both structures were solved by direct methods using
SHELXS97 (29) and difmap synthesis (SHELXL96) (30).
The refinements were done on F2 by full-matrix least
squares. All non-hydrogen atoms were refined anisotro-
pically, while the hydrogens (isotropic) were constrained to
the parent atom using a riding model. Crystal data and ex-
perimental details for 2·BH3 and 3 are listed in Table 1, and
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