A novel transformation of a zirconium imido compound and the development of a new class of N3 donor heteroscorpionate ligand

Article abstract of DOI:10.1039/b513927a

Reaction of the dimeric zirconium imido compound [Zr₂(μ-NAr) ₂Cl₄(THF)₄] with tris(3,5-dimethylpyrazolyl) methyl silane very selectively gave [Zr{(Me₂pz)₂Si(Me)NAr} Cl₃] (1), a highly active pre-catalyst for ethylene polymerisation; a more general and versatile route to N₃ donor heteroscorpionate compounds was achieved via the protio ligand (Me₂pz) ₂CHSi(Me)₂N(H)ⁱPr for which neutral and cationic organometallic Group 3 and 4 derivatives are reported (Ar = 2,6-C ₆H₃ⁱPr₂). The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513927a

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
A novel transformation of a zirconium imido compound and the
development of a new class of N3 donor heteroscorpionate ligand{
Robert G. Howe, Cara S. Tredget, Sally C. Lawrence, Suparabhorn Subongkoj, Andrew R. Cowley and
Philip Mountford*
Received (in Cambridge, UK) 30th September 2005, Accepted 25th October 2005
First published as an Advance Article on the web 21st November 2005
DOI: 10.1039/b513927a
transition metals in particular.13–15 Therefore a monoanionic, fac
Reaction of the dimeric zirconium imido compound
coordinating N3 amide-based ligand would be a valuable addition
to the rather limited range of monoanionic N3 donors so far
employed in early transition metal chemistry.15,16 The presently
available systems are:16 the pendant arm functionalised amidinates
(which have a non-constrained coordination geometry (fac or mer)
and can be prone to donor arm decoordination and ligand
redistribution for the early transition metals); bis(imino)pyrrolyl
ligands which enforce a mer coordination geometry; and the
ubiquitous tris(pyrazolyl)borates which can be prone to facile
displacement in Group 3, while in Group 4 (despite giving rise to
highly active ethylene polymerisation catalysts upon activation
with methyl aluminoxane (MAO)17) they are susceptible to very
facile B–N bond cleavage reactions.18
[Zr2(m-NAr)2Cl4(THF)4] with tris(3,5-dimethylpyrazolyl)methyl
silane very selectively gave [Zr{(Me2pz)2Si(Me)NAr}Cl3] (1), a
highly active pre-catalyst for ethylene polymerisation; a more
general and versatile route to N3 donor heteroscorpionate
compounds was achieved via the protio ligand
(Me2pz)2CHSi(Me)2N(H)iPr for which neutral and cationic
organometallic Group 3 and 4 derivatives are reported (Ar =
i
2,6-C6H3 Pr2).
Poly(pyrazolyl) based (‘‘scorpionate’’) ligands (see Chart 1) are
widely exploited over a broad range of transition metal applica-
tions including organometallic, catalytic, bioinorganic and supra-
molecular contexts.1–6 The axially symmetric anionic
tris(pyrazolyl)borates (e.g., I)3,4 and their neutral analogues the
tris(pyrazolyl)alkanes (e.g., II)1,5 are complemented by the anionic
‘‘heteroscorpionates’’ of type III in which a neutral pyrazolyl ring
has been formally substituted by an ‘‘arm’’ bearing an anionic
donor ‘‘X’’ which is almost exclusively a CS2, CO2, aryloxide or
alkoxide functional group.2,6,7 Despite the extensive later transition
metal literature for heteroscorpionates III, there are only a few
reports of the use of such ligands in Group 4,8–11 and only one of
these features any applications in either organometallic chemistry
or catalysis.8 Only one (very recent) report of the use of N2O or
N2S heteroscorpionates in Group 3 coordination chemistry has
appeared.12
In this contribution we report the unique transformation of a
bridging imido group into a new class of heteroscorpionate ligand
(the complex of which is a highly active ethylene polymerisation
catalyst). We also describe the rational, modular synthesis of
another type of N3 donor heteroscorpionate ligand and its neutral
and cationic early transition metal derivatives.
The pyrazolyl silane ligand MeSi(Me2pz)3 is a silicon analogue
of the tris(pyrazolyl)alkanes, but its coordination chemistry has
been very little studied (three reports to date, none from Group
4).5 The reaction of MeSi(Me2pz)3 with the dimeric zirconium
imido complex [Zr2(m-NAr)2Cl4(THF)4] (Ar = 2,6-C6H3 Pr2)19,20 is
i
summarised in eqn (1). After 3 h at room temperature in CH2Cl2
the heteroscorpionate compound [Zr{(Me2pz)2Si(Me)NAr}Cl3] (1)
was obtained in 62% isolated yield (assuming 1 equiv. of imido
dimer is required to form 1 equiv. of 1). The X-ray structure of 1
(Fig. 1){ is consistent with the solution NMR data and confirms
the presence of a fac-ZrCl3 fragment supported by a monoanionic
N3 donor heteroscorpionate ligand. The former imido group of
[Zr2(m-NAr)2Cl4(THF)4] forms the amide donor of the new ligand
which is the first in its class.
Although monoanionic N2O and N2S donor ligands III
themselves are extremely well known, no amide-based analogue
(X = anionic N donor) has yet been reported. Chelating di- and tri-
anionic polyamide ligands have been immensely important in
advancing the fundamental and catalytic chemistry of the early
ð1Þ
Chart 1 Important poly(pyrazolyl) ligands (illustrated for the more
common ring 3,5-disubstituted systems).
The reaction to form 1 was entirely reproducible and the same
heteroscorpionate product was also obtained in THF or benzene
as solvent or when 1 equiv. MeSi(Me2pz)3 was used per equiv. of
zirconium dimer. Compound 1 was the only identifiable product
Chemistry Research Laboratory, University of Oxford, Mansfield Road,
Oxford, UK OX1 3TA. E-mail: philip.mountford@chem.ox.ac.uk
{ Electronic supplementary information (ESI) available: characterising
data for the new compounds, polymer data and crystallographic data for 1
and 5. See DOI: 10.1039/b513927a
1
when the reaction was followed by H NMR spectroscopy. The
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 223–225 | 223

View Article Online
As shown in eqn (2), Me2SiCl2 reacts readily with
LiCH(Me2pz)2 to form (Me2pz)2CHSi(Me)2Cl (2) as a white solid
in 76% yield. Preliminary results show that 2 reacts smoothly with
an excess of a range of primary alkyl amines, but here we focus on
the reaction with iPrNH2 which afforded (Me2pz)2CHSi-
(Me)2N(H)iPr (3) as a pale oil in 74% yield on a gram scale (the
yield for neither 2 nor 3 has so far been optimised). Scheme 1
summarises preliminary studies of the reactions of 3 with various
Group 3 and 4 metal alkyls. Alkane or amine elimination reactions
are frequently the routes of choice to early transition metal
organometallic and similar complexes as the precursors are readily
available and ‘‘ate’’ complex formation is avoided, and so we have
focused on these protocols.
Reaction of protio ligand 3 with [Zr(CH2SiMe3)4] formed the
six-coordinate
tris(alkyl)
compound
[Zr{(Me2pz)2CHSi-
(Me)2NiPr}(CH2SiMe3)3] (4) which shows two inequivalent
1
CH2SiMe3 ligand environments in its H NMR spectrum at low
Fig. 1 Molecular structure of [Zr{(Me2pz)2Si(Me)NAr}Cl3] (1).
temperature. Reaction with [Sc(CH2SiMe3)3(THF)2] gave the five-
coordinate product [Sc{(Me2pz)2CHSi(Me)2NiPr}(CH2SiMe3)2]
(5), whereas with [Y(CH2SiMe3)3(THF)2] another six-coordinate
complex, namely [Y{(Me2pz)2CHSi(Me)2NiPr}(CH2SiMe3)2-
(THF)] (6) was obtained. The air- and moisture-sensitive
compounds are stable for many days in solution under an inert
atmosphere. The X-ray structure of 5 has been determined as
shown in Fig. 2.{ The new N3 donor heteroscorpionate ligand
adopts the expected fac coordination mode and the overall
geometry at scandium is square base pyramidal. The structure of 5
and NMR data for 6 suggest that the THF donor in the latter lies
trans to the amide donor.
fate of the ‘‘Zr(NAr)Cl(Me2pz)’’ fragment required by mass
balance is unknown at this time, as is the mechanism of this
remarkable transformation. Terminal Group 4 imido compounds
(i.e. having a non-bridging, multiply-bonded linkage MLNR) are
known to couple with a range of unsaturated linkages and also to
activate C–H and other element–hydrogen bonds.21,22 While the
net Si–N bond activation by a bridging imido ligand in
[Zr2(m-NAr)2Cl4(THF)4] is unique in this context, it is reminiscent
of the recently reported Me2pz ligand displacement from boron by
CH2Ph in [Zr{HB(Me2pz)3}(CH2Ph)2]+.18
The novel heteroscorpionate compound 1 is formally related to
Jordan’s tris(pyrazolyl)borate-supported trichlorides [Zr{HB-
(R3R5pz)3}Cl3]17 and Grassi’s N2O donor heteroscorpionate
[Zr{HC(Me2pz)2(C6H2(tBu)2O)}(CH2Ph)3]8 which are both olefin
polymerisation precatalysts. We found that when activated at room
temperature with MAO,§ 1 is a very highly active polymerisation
catalyst and forms polyethylene with a productivity in excess of
3120 kg(PE) mol21 h21 bar21 (Mw = 432,500; Mw/Mn = 2.5)
accompanied by a very rapid exotherm. This productivity is more
than an order of magnitude superior to that of the Grassi system at
this temperature, and is comparable to the MAO-activated
tris(pyrazolyl)borate systems for similar run times.17
The results obtained for 1 conclusively establish the new class of
N3 donor heteroscorpionate ligand as a viable contender with other
monoanionic scorpionate systems presently available. However,
although a number of different dimeric imido compounds
[Zr2(m-NR)2Cl4(L)n] are available in the literature,20 the scope of
the chemistry starting from these systems would inevitably be
limited. Therefore, encouraged by the results for 1, we developed a
new and potentially highly flexible synthesis of N3 donor hetero-
scorpionate ligands as outlined in eqn (2). This route allows for
modification of the pyrazolyl ring- and amido N-substituents, both
key variables based on what is already known about poly(pyrazolyl)
ligand1–6 and amide donor chemistry.13,15
ð2Þ
Scheme 1 Synthesis of Group 3 and 4 complexes of the N3 donor
heteroscorpionate ligand 3. Anion omitted for 7+ and 8+.
224 | Chem. Commun., 2006, 223–225
This journal is ß The Royal Society of Chemistry 2006

View Article Online
We thank the EPSRC and Thai Government for support, DSM
Research for gifts of [CPh3][BArF4] and Albermarle for MAO.
Notes and references
{ Crystal data: for C27H42Cl3N5OSiZr (1?THF), M = 678.33, orthorhom-
˚
bic, space group Pnma, a = 20.1923(7), b = 17.1332(6), c = 9.1763(3) A, V =
3174.62(19) A , T = 150 K, Z = 4, m = 0.665 mm21, 18858 reflections
3
˚
measured, 4056 unique, Rint = 0.068. Final R values R1 = 0.0400 and Rw
0.0471 (for I . 3s(I)); for C24H50N5ScSi (5), M = 537.91, triclinic, space
=
¯
˚
group = P1, a = 10.4827(2), b = 10.9399(3), c = 14.6107(4) A, a =
82.0937(10), b = 89.6905(11), c = 72.2736(12)u, T = 150 K, Z = 2, m =
0.367 mm21, 27522 reflections measured, 7134 unique, Rint = 0.047. Final
R values R1 = 0.0383 and Rw = 0. 0444 (for I . 3s(I)). CCDC 285656 and
285657. For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513927a
§ Polymerisation conditions: For 1, 6 bar C2H4, 2.5 mmol, 1500 equiv.
MAO, 250 mL toluene, 10 min. For 4, 6 bar C2H4, 10 mmol 4, 1 equiv.
[CPh3][BArF4], 5 mmol AliBu3, 250 mL toluene, 10 min.
Fig. 2 Molecular
(CH2SiMe3)2] (5).
structure
of
[Sc{(Me2pz)2CHSi(Me)2NiPr}-
1 H. R. Bigmore, S. C. Lawrence, P. Mountford and C. S. Tredget,
Dalton Trans., 2005, 635.
2 A. Otero, J. Fernandez-Baeza, A. Antinolo, J. Tejeda and A. Lara-
Sa´nchez, Dalton Trans., 2004, 1499.
3 S. Trofimenko, Scorpionates: the coordination chemistry of polypyrazo-
lylborate ligands, Imperial College Press, London, 1998.
4 S. Trofimenko, Polyhedron, 2004, 23, 197.
Bearing in mind that tris(pyrazolyl)borate-supported zirconium
dialkyl cations rapidly undergo N3 donor ligand degradation
above 0 uC18 we carried out the reaction between 4 and
1
[CPh3][BArF4] in CD2Cl2 (ArF = C6F5). The H and 19F NMR
spectra of the product mixture showed the formation of a stable
5 C. Pettinari and R. Pettinari, Coord. Chem. Rev., 2005, 249, 525.
6 C. Pettinari and R. Pettinari, Coord. Chem. Rev., 2005, 249, 663.
7 Very recently a lithium salt and zirconium trichloride derivative of a
cyclopentiadienyl-functionalised heteroscorpionate ligand was commu-
nicated, but no evidence of catalytic potential accompanied this report:
A. Otero, J. Ferna´ndez-Baeza, A. Antinolo, J. Tejeda, A. Lara-Sa´nchez,
L. Sa´nchez-Barba, A. M. Rodr´ıguez and M. A. Maestro, J. Am. Chem.
Soc., 2004, 126, 1330.
(for at least several hours at room temperature) dialkyl cation
[Zr{(Me2pz)2CHSi(Me)2NiPr}(CH2SiMe3)2]+ (7+) and separated
2
[BArF
]
4
anion. This suggested that such alkyl cations may be
viable olefin polymerisation catalysts. Subsequent evaluation of 4
with [CPh3][BArF4] cocatalyst in toluene in the presence of
AliBu3 gave an ethylene polymerisation productivity of
315 kg(PE) mol21 h21 bar21 (Mw = 458,000; Mw/Mn = 4.9).§
Recently, rare earth alkyl cations have gained much attention.23
Preliminary NMR tube scale studies showed that reaction of 5
with [CPh3][BArF4] in the presence of THF formed the stable (in
solution) monoalkyl cation [Sc{(Me2pz)2CHSi(Me)2NiPr}-
(CH2SiMe3)(THF)]+ (8+). Addition of ethylene to the NMR
sample afforded a white precipitate of polyethylene. Further
studies are presently under way.
8 S. Milione, S. Montefusco, T. Cuenca and A. Grassi, Chem. Commun.,
2003, 1176.
9 A. Otero, J. Fernandez-Baeza, A. Antinolo, F. Carrillo-Hermosilla,
J. Tejeda, A. Lara-Sa´nchez, L. Sa´nchez-Barba, J. Ferna´ndez-Lo´pez,
A. M. Rodr´ıguez and I. Lo´pez-Solera, Inorg. Chem., 2002, 41, 5193.
10 A. Otero, J. Fernandez-Baeza, A. Antinolo, J. Tejeda, A. Lara-Sanchez,
L. Sa´nchez-Barba, J. Ferna´ndez-Lo´pez and I. Lo´pez-Solera, Inorg.
Chem., 2004, 43, 1350.
11 A. Otero, J. Fernandez-Baeza, A. Antinolo, J. Tejeda, A. Lara-Sa´nchez,
L. Sa´nchez-Barba and A. M. Rodr´ıguez, Dalton Trans., 2004, 3963.
12 A. Otero, J. Fernandez-Baeza, A. Antinolo, J. Tejeda, A. Lara-Sanchez,
L. Sa´nchez-Barba, E. Mart´ınez-Caballero, A. M. Rodr´ıguez and
I. Lo´pez-Solera, Inorg. Chem., 2005, 44, 5336.
In conclusion, we have discovered a unique transformation of a
zirconium imido compound giving the first member of a new class
of monoanionic fac N3 donor heteroscorpionate ligand. We have
established a second type of such ligand for early transition metal
and rare earth chemistry, and have also demonstrated its viability
in key areas of current interest including olefin polymerisation and
the generation of well-defined alkyl cations. The high-yielding and
modular synthesis of the protio ligand (Me2pz)2CHSi(Me)2-
N(H)iPr (3) allows for straightforward and systematic modification
of both the pyrazolyl ring- and amide N-substituents, which are
key variables for the future development of these systems. We are
presently exploring monoanionic N3 donor heteroscorpionate
chemistry for a range of early transition and rare earth metals and
their applications.
13 L. H. Gade, Chem. Commun., 2000, 173.
14 R. Kempe, Angew. Chem., Int. Ed., 2000, 39, 468.
15 V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283.
16 For N3 donor ligands in Group 3 and leading references see W. E. Piers
and D. J. H. Emslie, Coord. Chem. Rev., 2002, 233–234, 131.
17 K. Michiue and R. F. Jordan, Organometallics, 2004, 23, 460.
18 H. Lee and R. F. Jordan, J. Am. Chem. Soc., 2005, 127, 9384.
19 D. J. Arney, M. A. Bruck, S. R. Huber and D. E. Wigley, Inorg. Chem.,
1992, 31, 3749.
20 S. R. Dubberley, S. E. Evans, C. L. Boyd and P. Mountford, Dalton
Trans., 2005, 1448.
21 A. P. Duncan and R. G. Bergman, The Chemical Record, 2002, 2, 431.
22 N. Hazari and P. Mountford, Acc. Chem. Res., DOI: 10.1021/
AR030244z, in press.
23 For a recent review see: S. Arndt and J. Okuda, Adv. Synth. Catal.,
2005, 347, 339.
This journal is ß The Royal Society of Chemistry 2006
Chem. Commun., 2006, 223–225 | 225