Titanium and zirconium complexes supported by dipyrrolide ligands

Article abstract of DOI:10.1039/b208751k

The reactions between meso-disubstituted dipyrromethanes and titanium and zirconium amides and alkyls have generated the first examples of dipyrrolide complexes of Group 4 metals.

Full text of DOI:10.1039/b208751k

Titanium and zirconium complexes supported by dipyrrolide ligands†
Andrew Novak, Alexander J. Blake, Claire Wilson and Jason B. Love*
School of Chemistry, University of Nottingham, University Park, Nottingham, UK NG7 2RD.
E-mail: jason.love@nottingham.ac.uk; Fax: +44 115 9513563; Tel: +44 115 8467167
Received (in Cambridge, UK) 9th August 2002, Accepted 15th October 2002
First published as an Advance Article on the web 25th October 2002
The reactions between meso-disubstituted dipyrromethanes
and titanium and zirconium amides and alkyls have
generated the first examples of dipyrrolide complexes of
Group 4 metals.
coordination mode to each zirconium centre, resulting in a
bimetallic structure. At first glance, each zirconium adopts a
distorted trigonal bipyramidal geometry, with the pyrrolide and
one benzyl ligand axial and the remaining benzyl and N
interactions equatorial. This ligation mode is common in
lanthanide chemistry, although the larger coordination number
exhibited by the lanthanides allows for expansion to less
predictable, macrocyclic structures.4 7 Closer analysis of the
bond lengths and angles between the zirconium and the
dipyrrolide and benzyl ligands indicate that the bonding mode
The dipyrromethane group (L), as well as being an important
precursor to porphyrinogens,1 and expanded and asymmetric
porphyrins,2 can act as a versatile, dianionic ligand due to
1
5
bimodal k /h bonding capability of the pyrrolide moieties.3 In
f-element chemistry, this feature promotes intricate aggregation
of reactive metal species.4,5 For example, macrocyclic clusters
of SmII[L] mediate the remarkable four-electron reduction of
nitrogen via the cooperative, one-electron oxidation of four
SmII centres.6,7 Given the rich chemistry of f-element dipyrro-
lide compounds and the similarity of L to ansa-cyclopentadie-
nyls,8 it is surprising that this ligand has received scant attention
in transition metal chemistry; to date, only copper-mediated
asymmetric catalysis using the planar-chiral dipyrromethane
1
5
of L1 is not straightforward k /h . While the benzyl ligands
2
attached to Zr(2) are classically bound, a weak h -interaction
between Zr(1) and benzyl C(12) exists [Zr(1)–C(12)–C(13)
2
101.4(2), Zr(1)…C(13) 2.952 Å]. This coincides with an h -
pyrrolide interaction at Zr(1) [Zr(1) to N(1) and C(4) are
significantly shorter than Zr(1) to C(1), C(2) and C(3)],
3
compared to the h -pyrrolide ligation to Zr(2). It is therefore
evident that L1 can adopt coordination modes that satisfy the
electronic requirements of the metal.
5
5
ligand [(h -C4H3N)(h -Cp*Fe)]2CH2 has been reported.9 Here,
we describe selected protonolysis and transamination reactions
between the meso-disubstituted dipyrromethanes L and zirco-
nium and titanium alkyls and amides (see Scheme 1). To our
knowledge, the compounds described below are the first Group
4 dipyrrolide compounds and rare examples of dipyrrolide
transition metal compounds.
The transamination reaction between Ti(NMe2)4 and L1 (or
1
L2) was monitored by H NMR spectroscopy and, unlike the
zirconium reaction above, proceeds to generate the orange,
monometallic, titanium complex 2 in quantitative yield upon
mixing at room temperature.† As with 1, limited structural
information is available from the 1H NMR spectrum, so an X-
ray diffraction study was undertaken (Fig. 2).‡ In this case, the
The reaction between L1 and 2 molar equivalents of
Zr(CH2Ph)4 at 50 °C was monitored by 1H NMR spectroscopy,
and was found to generate the orange, bimetallic zirconium
complex 1 in quantitative yield after 2.5 h; prolonged reaction
times or heating to higher temperatures resulted in substantial
1
5
dipyrrolide ligand is seen to adopt a unique k :h -bonding mode
to the single titanium, so generating an overall psuedo-
tetrahedral geometry that is reminiscent of constrained geome-
try catalysts (CGCs).10 Indeed, the N(2)–Ti(1)-centroid angle of
1
5
decomposition.† The H NMR spectrum is structurally unin-
104.4° is similar to that seen for the archetypal CGC [h -
formative; at room temperature, the benzyl CH2 protons
resonate as a single AB doublet. Therefore, in order to assess the
binding mode of the ligand, an X-ray diffraction study was
undertaken (Fig. 1).‡ The dipyrrolide ligand assumes a s/p
C5Me4SiMe2NBut]TiCl2 (105.50°) and that of the mixed Cp–
5
1
pyrrolide
titanium
complex
[h -C5H4CH2-k -
C4H3N]Ti(NMe2)2 (103°).11 In contrast to the CGC systems,
the presence of the nitrogen in the pyrrolic ring reduces the
Scheme 1 Reagents and conditions: (i) Zr(CH2Ph)4, PhMe, 50 °C, 2.5 h; (ii)
Ti(NMe2)4, PhMe; (iii) Zr(NEt2)4, PhMe; (iv) ClSiMe3, PhMe.
† Electronic supplementary information (ESI) available: Full experimental,
NMR and analytical data. See http://www.rsc.org/suppdata/cc/b2/
b208751k/
Fig. 1 The molecular structure of 1. Selected bond lengths (Å): Zr(1)–N(2)
2.330(2), Zr(1)–N(1) 2.465(3), Zr(1)–C(4) 2.473(3), Zr(1)–C(1) 2.633(3),
Zr(1)–C(2) 2.732(3), Zr(1)–C(3) 2.642(3).
2796
CHEM. COMMUN., 2002, 2796–2797
This journal is © The Royal Society of Chemistry 2002

symmetry of the p-bound ligand and so renders 2 intrinsically
chiral. However, while the asymmetric unit of the solid state
structure shows two molecules of the same hand, the overall
structure is centrosymmetric and so confirms that 2 has
crystallised as a racemic mixture. In solution, a dynamic process
renders both sets of pyrrolic and Me2N groups equivalent, a
process that must involve facile, thermally induced, pyrrolic
hapticity changes.12 Unlike cyclopentadienyl-based Group 4
amido complexes where the amido group is substituted by
chloride on addition of chlorosilane, 2 undergoes a con-
proportionation reaction when treated with ClSiMe3 to form the
bis(dipyrrolide) complex 5 as a dark red, crystalline material. X-
Ray structural analysis confirmed this formulation,† with both
structural motifs upon ligation that appear dependent on the
electronic/coordinative requirements of the metal. Importantly,
some control over complex formation is possible, generating
monometallic amido and bimetallic alkyl Group 4 compounds,
and not the intricate clusters as seen for Yb and Sm. At present,
we are investigating the reactivity of these compounds towards
small molecules and are exploring routes to the elaboration of
the dipyrrolide ligand periphery.
We thank the Royal Society (J. B. L., University Research
Fellowship), the Nuffield Foundation (A. N., Undergraduate
Research Bursary), and the University of Nottingham for
support, Prof. F. G. N. Cloke and Dr G. K. B. Clentsmith for
help with the ethene polymerisation studies, the EPSRC/Rapra
Technology for the polymer analysis and the EPSRC for the
award of the diffractometer.
1
5
dipyrrolide ligands adopting similar k /h bonding modes to 2
1
(see Scheme 1). In the H NMR spectrum of 5, a single set of
resonances are seen for the dipyrrolide protons, indicating that,
as with 2, a dynamic process that equilibrates the pyrrolic rings
is occurring.
Notes and references
In order to determine if the reaction pathway forming
bimetallic 1 or monometallic 2 is dependant on the metal radius
or on the type of ancillary ligand (amido or alkyl), the reactions
between L1 and Zr(NEt2)4 and Ti(CH2SiMe3)4 were monitored
by NMR spectroscopy. The former reaction in C6D6 showed
that a 1+1 reaction occurs, and that the evolved HNEt2 was
easily removed under vacuum; presumably 4 has a similar
structure to 2. The latter protonolysis reaction was more
problematic; a slow reaction occurred at elevated temperatures
(100 °C) yielding a paramagnetic substance that has yet to be
characterised.
As the dipyrrolide complexes described above have structural
similarity to CGCs, a preliminary alkene polymerisation study
was undertaken. While the titanium compounds 2 and 5 showed
negligible activity as ethene polymerisation catalysts,† pre-
treatment of 4 with ClSiMe3 followed by MAO gave a species
that promoted ethene polymerisation with an activity of 8.7 kg
mol21 h21 bar21 (cf. 29.2 kg mol21 h21 bar21 using Cp2ZrCl2
under identical conditions). Analysis of this polymer by high
temperature gel permeation chromatography showed that it was
of very high molecular weight (Mw = 1.2 3 106) and high
polydispersity (PD = 80) (cf. Mw = 1.27 3 104, PD = 3.9 for
Cp2ZrCl2).
‡ Crystal data: [Zr2(CH2Ph)6(L1)] 1, orange tablet, 0.17 3 0.12 3 0.08
mm3, C53H54N2Zr2, monoclinic, space group P21/c, a = 18.8621(13), b =
11.2299(8), c = 20.6506(14) Å, b = 96.096(2)°, U = 4349.5(5) Å3, Z =
4, m = 0.517 mm21, F(000) = 1864, 33815 collected reflections, 7620
unique (Rint = 0.055). Data were collected at 150(2) K on a Bruker
SMART1000 CCD, l
= 0.71073 Å, q = 1.98–25.00°, absorption
correction applied using SADABS, solved by direct methods and refined
using SHELXL-97. Final full-matrix least-squares refinement on F2
converged at R1 = 0.0343 for 7620 reflections with I > 2s(I), wR2
=
0.0637, S = 1.043 for all data and 514 parameters. [Ti(NMe2)2(L1)] 2,
orange lozenge, 0.41 3 0.38 3 0.18 mm3, C15H24N4Ti, monoclinic, space
group P21/c, a = 10.3274(6), b = 12.3793(7), c = 25.3112(14) Å, b =
93.705(1)°, U = 3229.2(3) Å3, Z = 8, m = 0.527 mm21, F(000) = 1312,
19861 collected reflections, 7881 unique (Rint = 0.021), l = 0.71073 Å, q
= 1.83–28.72°, absorption correction applied using SADABS, solved by
direct methods and refined using SHELXL-97. Final full-matrix least-
squares refinement on F2 converged at R1 = 0.0323 for 7536 reflections
with I > 2s(I), wR2 = 0.0799, S = 1.026 for all data and 369 parameters.
CCDC 193472 and 193473. See http://www.rsc.org/suppdata/cc/b2/
b208751k/ for crystallographic data in CIF or other electronic format.
1 C. Floriani, Chem. Commun., 1996, 1257; B. Turner, A. Shterenberg, M.
Kapon, K. Suwinska and Y. Eichen, Chem. Commun., 2002, 404.
2 A. Jasat and D. Dolphin, Chem. Rev., 1997, 97, 2267; C.-H. Lee and J.
S. Lindsey, Tetrahedron, 1994, 50, 11427.
3 E. Campazzi, E. Solari, R. Scopelliti and C. Floriani, Inorg. Chem.,
1999, 38, 6240; J. M. Tanski and G. Parkin, Organometallics, 2002, 21,
587; M. Ganesan, C. D. Bérubé, S. Gambarotta and G. P. A. Yap,
Organometallics, 2002, 21, 1707.
We have shown that meso-disubstituted dipyrromethane
ligands are suitable for Group 4 metals, and can adopt various
4 T. Dubé, S. Conoci, S. Gambarotta and G. P. A. Yap, Organometallics,
2000, 19, 1182.
5 D. M. M. Freckmann, T. Dube, C. D. Berube, S. Gambarotta and G. P.
A. Yap, Organometallics, 2002, 21, 1240.
6 T. Dubé, S. Conoci, S. Gambarotta, G. P. A. Yap and G. Vasapollo,
Angew. Chem., Int. Ed., 1999, 38, 3657.
7 M. Ganesan, S. Gambarotta and G. P. A. Yap, Angew. Chem., Int. Ed.,
2001, 40, 766.
8 H. H. Brintzinger, D. Fischer, R. Mulhaupt, B. Rieger and R. M.
Waymouth, Angew. Chem., Int. Ed., 1995, 34, 1143; C. E. Zachmano-
glou, A. Docrat, B. M. Bridgewater, G. Parkin, C. G. Brandow, J. E.
Bercaw, C. N. Jardine, M. Lyall, J. C. Green and J. B. Keister, J. Am.
Chem. Soc., 2002, 124, 9525.
9 M. M.-C. Lo and G. C. Fu, J. Am. Chem. Soc., 2002, 124, 4572; M. M.-
C. Lo and G. C. Fu, J. Am. Chem. Soc., 1998, 120, 10270. Note added
at proof: tantalum dipyrrolide compounds have been recently reported:
G. Aharonian, S. Gambarotta and G. P. A. Yap, Organometallics, 2002,
21, 4247.
10 A. L. McKnight and R. M. Waymouth, Chem. Rev., 1998, 98, 2587.
11 W. S. Seo, Y. J. Cho, S. C. Yoon, J. T. Park and Y. Park, J. Organomet.
Chem., 2001, 640, 79; D. W. Carpenetti, L. Kloppenburg, J. T. Kupec
and J. L. Petersen, Organometallics, 1996, 15, 2668.
12 D. P. Heenan, C. Long, V. Montiel-Palma, R. N. Perutz and M. T. Pryce,
Organometallics, 2000, 19, 3867.
Fig. 2 The molecular structure of 2 (one molecule from the asymmetric unit
is shown). Selected bond lengths (Å): Ti(1)–N(2) 2.0389(13), Ti(1)–N(3)
1.8963(13), Ti(1)–N(4) 1.8866(13), Ti(1)–cent. 1.700.
CHEM. COMMUN., 2002, 2796–2797
2797