A strange nickel(I)-nickel(0) binuclear complex and its unexpected ethylene oligomerization

Article abstract of DOI:10.1021/om0701322

The unusual heterovalent bimetallic Ni¹-Ni⁰ complex {[η-C₅H₄CH=N(C₆F₅)]Fe[η- C₅H₄PPh₂]}₂Ni₂(Cl) (2) was isolated from the reaction between [η-C₅H₄CH= N(C₆F₅)]Fe[η-C₅H₄PPh ₂] (1) and Ni(COD)₂ in the presence of AlEtCl₂ or AlCl₃. Complex 2 has been crystallographically characterized and theoretically analyzed; it is catalytically active (TOF 90 500) and selective (91%) toward the formation of C₄ oligomers from ethylene (300 psi) at 30 °C, the only other significant isomer formed being C₆.

Full text of DOI:10.1021/om0701322

2950
Organometallics 2007, 26, 2950-2952
A Strange Nickel(I)-Nickel(0) Binuclear Complex and Its
Unexpected Ethylene Oligomerization
Zhiqiang Weng,*,† Shihui Teo,† Zhi-Pan Liu,*,‡ and T. S. Andy Hor*,†
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3, Kent Ridge,
Singapore 117543, and Department of Chemistry, Fudan UniVersity, Shanghai 200433,
People’s Republic of China
ReceiVed February 10, 2007
Summary: The unusual heteroValent bimetallic NiI-Ni0 complex
{[η-C5H4CHdN(C6F5)]Fe[η-C5H4PPh2]}2Ni2(Cl) (2) was iso-
lated from the reaction between [η-C5H4CHdN(C6F5)]Fe[η-
C5H4PPh2] (1) and Ni(COD)2 in the presence of AlEtCl2 or
AlCl3. Complex 2 has been crystallographically characterized
and theoretically analyzed; it is catalytically actiVe (TOF
90 500) and selectiVe (91%) toward the formation of C4
oligomers from ethylene (300 psi) at 30 °C, the only other
significant isomer formed being C6.
We have recently isolated the rare three-coordinate NiI
complex [η-C5H4CHdN(C6H5)]Fe[η-C5H4P(t-Bu)2]NiICl and
the unusual bimetallic Ni0-AlIII complex [η-C5H4CHdN(C6F5)]-
Fe[η-C5H4PPh2]Ni(AlMe3).9 These successes have encouraged
us to extend the use of unsaturated metals carrying a hybrid
ligand8 to design new low-valent Ni catalysts. In this com-
munication, we report the isolation of an intriguing NiI-Ni0
bimetallic complex that is more active than its NiII and Ni0
counterparts. Although bimetallic catalysts have exhibited some
advantages in certain systems such as selective hydrogenation,10
asymmetric epoxide opening reactions,11 and copolymeriza-
tion,12 the use of such a system in ethylene oligomerization is
still less developed.13
Reaction between Ni0(COD)2 (COD ) 1,5-cyclooctadiene)
and the hybrid ligand [C5H4CHdN(C6F5)]Fe(η-C5H4PPh2) (1)
in toluene in the presence of a polymerization promoter such
as AlEtCl2 or AlCl3 gives 57% yield of an air- and moisture-
sensitive red solid formulated as {[η-C5H4CHdN(C6F5)]Fe[η-
C5H4PPh2]}2Ni2Cl (2) (eq 1). The solid form is fairly stable
(for days) under an inert atmosphere at room temperature, while
its solution shows minimum decomposition after 2 days at -30
°C. Complex 2 is unusual. The oxidation self-selects to stop at
the stage of a binuclear NiI-Ni0 species. We also could not
detect any binuclear NiI or any major NiII or Ni0 products under
the experimental conditions. Its paramagnetism frustrates any
reasonable NMR analyses. In view of its unusual formulation,
Nickel is the “disturbing” element in Ziegler’s Nobel-winning
work in ethylene polymerization.1 Its salt is the source of the
“nickel effect” that accounts for the premature polymer termina-
tion.2 Today, five decades later, research in this area has led to
the development of Ni-based systems that catalyze the growth
of the C6-C20 linear R-olefin (LAO) industry.3 Inspired by the
SHOP process,4 different teams (e.g., Brookhart,5 DuPont,6
Grubbs,7 Braunstein,8 etc.) have designed a number of elegant
olefin oligomerization catalysts. Notable challenges remain,
however, such as activity under ambient conditions, selectivity
over specific oligomers (e.g., C6 LAO), ambiguous mechanistic
details, etc. Interestingly, virtually all known Ni-based LAO
catalysts are NiII systems. The role of low-valent nickel in this
technologically significant process is virtually unknown. Our
objective is to design and synthesize suitable low-valent Ni
model complexes that could help in the promotion and
understanding of Ni-based ethylene oligomerization.
* To whom correspondence should be addressed. E-mail: chmwz@
nus.edu.sg (Z.W.); andyhor@nus.edu.sg (T.S.A.H.).
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† National University of Singapore.
‡ Fudan University.
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10.1021/om0701322 CCC: $37.00 © 2007 American Chemical Society
Publication on Web 05/05/2007

Communications
Organometallics, Vol. 26, No. 12, 2007 2951
Figure 1. Molecular structure of complex 2 (phenyl rings on the
P and H atoms are omitted). Selected bond lengths (Å) and angles
(deg): Ni(1)‚‚‚Ni(2) ) 2.9607(13), Ni(1)-C(1) ) 1.901(7), Ni-
(1)-N(2) ) 1.934(6), Ni(1)-N(1) ) 1.959(6), Ni(1)-P(1) )
2.141(2), Ni(2)-C(2) ) 1.904(7), Ni(2)-N(2) ) 1.940(6), Ni(2)-
P(2) ) 2.148(2), Ni(2)-Cl(1) ) 2.265(2), N(1)-C(1) ) 1.453(9),
N(2)-C(2) ) 1.455(9); C(1)-Ni(1)-N(1) ) 44.2(3), C(2)-Ni-
(2)-N(2) ) 44.5(3).
we carried out an X-ray single-crystal diffraction to establish
its structure.14 Suitable single crystals were obtained by diffusion
of n-hexane into its CH2Cl2 or toluene solution under argon.
The structure (Figure 1) shows a dinickel core with a CNπ,P-
iminophosphine chelate on one nickel and terminal chloride on
another. The two metals are brought into close proximity (Ni-
Ni ) 2.9607(13) Å) by a second iminophosphine that shows
its maximum donation capacity as a 6e donor (2e P + 2e π CN
on one metal and 2e σ N on the other) that bridges the metals.
Ignoring the Ni-Ni bond, both metals adopt a distorted-square-
planar geometry (the dihedral angle between Ni(1)-C(1)-N(1)
and Ni(1)-N(2)-P(1) is 6°, and that between Ni(2)-C(2)-
N(2) and Ni(2)-Cl(1)-P(2) is 1.9°). The two C6F5 substituents
are oriented on the top half of the molecule, showing some
degree of π···π stacking (interplanar distance 3.34 Å). The two
phosphines of 2 show good donicity, giving two strong Ni-P
bonds (2.148 and 2.141 Å) which are shorter than the usual
Ni-P bonds (2.151-2.169 Å).8d,9,15 The Ni-Cl bond (2.265
Å), on the other hand, is unexpectedly weak, being longer than
the regular NiI-Cl bonds (2.192-2.231 Å).8,9a,15 This appears
to be a common feature of the complexes formed from the
iminophosphine.9a The Ni-Ni separation (2.9607(13) Å) is
closer to the nonbonding interactions found in many binuclear
Ni(II) structures (2.973-3.064 Å)16 than to the conventional
metal-metal-bonded NiI-NiI (2.30-2.52 Å),17 Ni0-Ni0 (2.572-
2.603 Å),18 and NiII-NiII dimers (2.281-2.7527 Å).19
complex 2, it is dissected into two fragments: (a) NiICl-1 and
(b) Ni0-1. Interestingly, the calculations revealed that the
standalone Ni0-1 and NiICl-1 are also stable, with fully
optimized structures similar to their counterparts in complex 2.
The coupling of NiICl-1 with Ni0-1 will gain energy by 1.47
eV, indicating that the binuclear structure is more stable than
the mononuclear forms. The HOMO of binuclear 2 is singly
occupied, which is found to be antibonding between the Ni
halves but bonding within the Ni0-1 moiety. This implies that
adding an electron to 2 would destabilize the binuclear structure,
whereas extracting an electron would weaken the internal bond
of the Ni0-1 portion. Therefore, it appears that the presence of
NiINi0 instead of Ni0Ni0 or NiIINi0 is due to the bifunctionality
of the HOMO of 2: a half-occupation in this orbital is most
thermodynamically favored.
We now turn to determining how the neutral ligand 1 bonds
with Ni and how the “weak” imine holds the two metals together
and stabilize both. Our calculations revealed that 1 is a metallo
ligand with a large LUMO-HOMO gap of 1.89 eV. Its LUMO
is a CdN antibonding state, while the HOMO, HOMO-1, and
HOMO-2 are all d orbitals of Fe. The HOMO-3 and HOMO-4
are related to the nonbonding 2p orbital of P and the π bonding
state of CdN. For the formation of the Ni0-1 fragment, we
With two formally neutral iminophosphines and a chloride,
this is formally a NiI-Ni0 binuclear complex, which is unusual.
The presence of unsaturated and paramagnetic metal centers in
a complex that depends so much on ligand support from a
seemingly weak donor (imine) is also surprising. To gain some
insights into these structural anomalies, we have performed
density functional theory (DFT)20 calculations. To analyze
(17) (a) Chen, Y.; Sui-Seng, C.; Zargarian, D. Angew. Chem., Int. Ed.
2005, 44, 7721. (b) Barnett, B. L.; Kru¨ger, C.; Tsay, Y. H. Chem. Ber.
1977, 110, 3900. (c) Jones, R. A.; Stuart, A. L.; Atwood, J. L.; Hunter, W.
E.; Rogers, R. D. Organometallics 1982, 1, 1721. (d) Fryzuk, M. D.;
Clentsmith, G. K. B.; Leznoff, D. B.; Rettig, S. J.; Geiib, S. J. Inorg. Chim.
Acta 1997, 265, 169. (e) Kriley, C. E.; Woolley, C. J.; Krepps, M. K.;
Popa, E. M.; Fanwick, P. E.; Rothwell, I. P. Inorg. Chim. Acta 2000, 300-
302, 200.
(18) (a) Ko¨nig, H.; Menu, M. J.; Dartiguenave, M.; Dartiguenave, Y.;
Klein, H. F. J. Am. Chem. Soc. 1990, 112, 535. (b) DeLeat, D. L.; Fanwick,
P. E.; Kubiak, C. P. Organometallics 1986, 5, 1807.
(19) (a) Kersting, B.; Siebert, D. Inorg. Chem. 1998, 37, 3820. (b)
Brooker, S.; Croucher, P. D.; Davidson, T. C.; Dunbar, G. S.; Beck, C. U.;
Subramanian, S. Eur. J. Inorg. Chem. 2000, 169. (c) Harrop, T. C.;
Olmstead, M. M.; Mascharak, P. K. Chem. Commun. 2003, 410. (d)
Trinquier, G.; Dartiguenave, M.; Dartiguenave, Y.; Be´nard, M. Inorg. Chem.
1991, 30, 4491.
(14) Crystal data for 2‚C6H14‚CH2Cl2: Mr ) 1450.51, triclinic, space
group P1h, a ) 12.867(2) Å, b ) 14.027(2) Å, c ) 17.800(3) Å, R )
100.472(3)°, â ) 106.307(3)°, γ ) 90.936(4)°, V ) 3024.2(8) Å3, Z ) 2,
F ) 1.593 Mg m-3, F(000) ) 1474, λ(Mo KR) ) 0.710 73 Å, µ ) 1.343
mm-1, T ) 223(2) K, crystal dimensions 0.34 × 0.20 × 0.18 mm. A
Siemens SMART diffractometer, equipped with a CCD detector, was used.
Of 25 829 reflections measured, 9040 unique reflections were used in the
refinement. Final R ) 0.0695 (Rw ) 0.1598). CCDC 627879.
(15) (a) Hou, J.; Sun, W.-H.; Zhang, S.; Ma, H.; Deng, Y.; Lu, X.
Organometallics 2006, 25, 236. (b) Sauthier, M.; Leca, F.; Souza, R. F.;
Bernardo-Gusmajo, K.; Queiroz, L. F. T.; Toupet, L.; Re´au, R. New J. Chem.
2002, 26, 630.
(16) (a) Kersting, B.; Siebert, D. Inorg. Chem. 1998, 37, 3820. (b)
Brooker, S.; Croucher, P. D.; Davidson, T. C.; Dunbar, G. S.; Beck, C. U.;
Subramanian, S. Eur. J. Inorg. Chem. 2000, 169. (c) Kersting, B. Eur. J.
Inorg. Chem. 1999, 2157. (d) Kersting, B.; Steinfeld, G.; Hausmann, J.
Eur. J. Inorg. Chem. 1999, 179.
(20) (a) Ziegler, T. Chem. ReV. 1991, 91, 651. (b) Parr, R. G.; Yang, W.
Density Functional Theory of Atoms and Molecules; Oxford University
Press: New York, 1989.

2952 Organometallics, Vol. 26, No. 12, 2007
Communications
Table 1. Activities toward Oligomerization of Ethylenea,b
(65 300), Ni(COD)2 + 1 in situ (64 800), and the Ni(0) tris-
(isocyanide) complex [{[η-C5H4CHdN(C6F5)]Fe[η-C5H4PPh2]}-
Ni0(CNtBu)3]9a (75 000). It is still unclear if such higher activity
could be related to the metal cooperative effect or to the ability
of 2 to dissociate into its catalytically active mononuclear forms
that are coordinatively unsaturated. Further experiments using
MAO or AlMe3 as activator also gave inconclusive evidence
on its exact role. We should add also that the differences are
less significant compared to those in the chromium system, in
which the Cr(II) tripyrrolide is more active than its Cr(III)
counterpart.23 Our systems show good and similar selectivities
toward C4 oligomer formation (90-93%), with C6 being the
only other notable isomers (7-10%). Selectivities toward R-C4
(26-27%) are also similar (Table 1, entries 1-3). These could
suggest similar active species. The preference for C4 and C6
over other oligomers or polymers is consistent with the facile
â-elimination being the key chain termination step. Use of
AlEtCl2 as an activator does not improve the activity of this
NiI-Ni0 complex (entries 4 and 5). These results are inconsistent
with those observed for our previous NiI and Ni0-Al complexes
as well as the [P,N]-chelating Ni catalytic systems,8 possibly
due to the differences in the Ni-alkyl species formed that
precede the active Ni-hydride.
Although we are unable to pinpoint how a bimetallic
heterovalent Ni2 species can promote olefin oligomerization, it
is clear that its performance is at least comparable with those
of its Ni0 and NiII analogues. There is no experimental or
theoretical evidence that the bimetallic 2 must undergo frag-
mentation for it to be catalytically active. Current work is
focused on the study of the role of its hybrid ligand and the
associated hemilability in determining the catalytic efficacy. The
present and earlier studies suggested that this ligand shows
different dimensions of coordination and donor flexibilities:
namely, 2e T 4e T 6e donor, P only T P + CN(π) T P +
CN(π) + N(σ) coordination, unidentate T chelating T chelate-
bridging mode, and low T intermediate T normal metal valency
stabilization. This is achieved by balancing its σ and π donation
with π acceptance behaviors. Such multidimensional flexibility
could help the ligand support and stabilize not only the
precatalyst but also the different intermediates along the catalytic
pathway. The isolation of 2 and other unusual Ni complexes
suggested that more unexpected structures can be expected.
oligomerd
C4/
∑C
C6/ R-olefin
entry
cat.
activator
MAO
TOFc
∑C
(C4)
1
2
90 500 91.2
65 300 90.1
64 800 93.4
8.8
9.9
6.6
27.3
26.4
26.4
22.2
24.5
2e
3
NiBr2(DME) + 1 MAO
Ni(COD)2 + 1
MAO
4
5
2f
EtAlCl2 87 300 78.8 21.2
EtAlCl2 58 000 91.3 8.7
Ni(COD)2 + 1
a Average of two runs. b Conditions: 0.2 µmol of catalyst (based on Ni),
4 mL of toluene, 30 °C, 300 psi of ethylene, Al/Ni ) 500, 1 h. c TOF )
(mol of ethylene consumed)/((mol of Ni) h)-1
.
d In units of mol %.
e Prepared in CH2Cl2. f 2 h.
observe that the d orbitals of Ni are split into two groups: group
2
2
I, dz , dxz, and dyz, which are largely nonbonding states, with dz
2
2
being the HOMO; group II, dxy and dx -y , which form covalent
bonds with the empty LUMO of 1 and the occupied p states of
P, respectively. Consistently, as Ni bonds with CN, the CN bond
is lengthened from ∼1.30 to ∼1.40 Å, indicating the weakening
of the CdN bond and π bonding. In the Ni0-1 molecular
fragment, the Ni is three-coordinated (with P, N, and C). It is
expected that the Ni can still bond with one extra electron-rich
ligand to complete the typical square-planar four-coordination.
This could be achieved by species such as Cl- and imine (the
lone-pair electron of N), which are able to donate an electron
to the empty 4s state of the Ni.
Cationic species have been proposed to be the active
components in olefin polymerization/oligomerization catalysis.21
Our calculations of the ionization energy and electron affinity
of 2 gave 6.01 and 2.13 eV, respectively. Interestingly, this
indicates that the conversion of 2 to ionic species is facile. The
calculation results also suggested that the ethylene molecule does
not promote the cleavage of the binuclear species into the
separated mononuclear form, as such a dissociation would
require 1.46 eV, which is more than the energy gain from
ethylene adsorption on separated monomers. Thus, DFT appears
to support the binuclear 2 to function as an intact molecular
cationic entity upon Cl dissociation and subsequent ethylene
adsorption and oligomerization.
Complex 2 was then examined for ethylene oligomerization
(300 psi) activity in toluene at 30 °C in the presence of MAO
as coactivator (Al/Ni ) 500).22 There is no apparent formation
of R-oligomers in the absence of MAO. Some comparisons of
the catalytic activities of 2 with those of its Ni(II) and Ni(0)
counterparts are summarized in Table 1. Preliminary experi-
ments showed that when it is activated by excess MAO under
similar conditions, the bimetallic NiI-Ni0 complex 2 gives
activities (TOF 90 500) somewhat higher than for its related
NiII and Ni0 counterparts, such as NiBr2(DME) + 1 in situ
Acknowledgment. Financial support from the National
University of Singapore and a scholarship to S.T. are gratefully
acknowledged. We are grateful to Dr. L. L. Koh and Ms. G. K.
Tan for assistance in the crystallographic analysis.
Supporting Information Available: Text, tables, figures, and
a CIF file giving experimental details and X-ray structural data for
2 and LUMO and HOMO orbitals of ligand 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
(21) (a) Chen, E. Y. X.; Marks, T. J. Chem. ReV. 2000, 100, 139. (b)
Erker, G. Dalton Trans. 2005, 1883.
(22) The experiments were carried out in a Biotage AB parallel pressure
reactor with the Endeavor Catalyst Screening System (http://www.biotage.
com).
OM0701322
(23) Crewdson, P.; Gambarotta, S.; Djoman, M.-C.; Korobkov, I.;
Duchateau, R. Organometallics 2005, 24, 5214.