Neutral and zwitterionic group 4 metal alkyls with ancillary boroxide ligands

Article abstract of DOI:10.1039/b414155e

The boroxide anion, [OB(mes)₂]^-, has been applied as an ancillary ligand to generate electron-deficient group 4 metal alkyls; however, the enhanced electrophilicity results in formation of tight ion-pairs in solution.

Full text of DOI:10.1039/b414155e

View Article Online / Journal Homepage / Table of Contents for this issue
C O M M U N I C A T I O N
Neutral and zwitterionic group 4 metal alkyls with ancillary
boroxide ligands
Sarah C. Cole, Martyn P. Coles* and Peter B. Hitchcock
The Department of Chemistry, University of Sussex, Falmer, Brighton, UK BN1 9QJ.
E-mail: m.p.coles@sussex.ac.uk; Fax: +01273 677196; Tel: +01273 877339
Received 14th September 2004, Accepted 28th September 2004
First published as an Advance Article on the web 7th October 2004
The boroxide anion, [OB(mes)2]−, has been applied as an
ancillary ligand to generate electron-deficient group 4 metal
alkyls; however, the enhanced electrophilicity results in
formation of tight ion-pairs in solution.
The quest for alternatives to the ubiquitous cyclopentadienyl
anion in early transition-metal olefin polymerisation catalysis
remains one of the most intensely studied areas of organo-
metallic chemistry.1 As such, numerous ligand sets have been
investigated as ancillary groups for this reactivity and many
elegant approaches have been developed which enable fine-
tuning of their steric and electronic properties. The chelating
diamido framework ranks amongst the most successful group
of ligands in this context, where, in addition to the nature of
the linking-group, the N-substituents are highly influential in
controlling the environment about the active-site. To reduce
electron-density at the metal by decreasing the extent of
p-donation, commonly associated with an increased activity,
Schrock and co-workers incorporated boryl-groups adjacent
to the donor nitrogen atom.2 Unfortunately, the effect was
too pronounced in these cases and resulted in the formation
of low-activity systems for olefin polymerisation, due to facile
decomposition pathways for the neutral dimethyl precursors
and strong anion binding in the cationic systems.
Fig. 1 Molecular structure of 2 (ellipsoids drawn at the 20% prob-
ability level, hydrogen atoms omitted). Selected bond lengths (Å) and
angles (°): Ti–O1 1.7965(18), Ti–O2 1.7923(18), Ti–O3 1.7918(19),
Ti–C55 2.081(3); Ti–O1–B1 162.57(19), Ti–O2–B2 163.6(3), Ti–O3–B3
160.60(18), Ti–C55–C56, 109.98(18).
donor pyridine ligands and the increased coordination number
in the imido complex. However, despite the formal low electron
count for Ti in compound 2, there is no evidence for any g2-
type interaction with the benzyl group [TiC56 = 2.942 Å;
Ti–C55–C56 = 109.98(18)°].
Recent work from our group3 has demonstrated that
alkoxide ligands substituted with boryl groups (referred to as
boroxides),4,5 have similar p-donor properties to substituted
aryloxide ligands, illustrated by solid- and solution-state studies
of transition-metal imido species. Since conventional aryloxides
have been moderately successful as ancillary ligands in active
catalysts for the polymerisation of a-olefins,6,7 we have initiated
a study of group 4 metal alkyls incorporating boroxide ligands
and their applications in ethylene polymerisation catalysis.
Alkyl-elimination reactions, employing dimesitylborinic
acid as the ligand source, were investigated as a straight-
forward method for the synthesis of target compounds with
A number of substituted phenoxide ligands have been
shown to form monomeric compounds with formula Ti(OAr)4
[Ar = 2,6-iPr2C6H3,9 2-tBuC6H4 and 2,3,5,6-Me4C6H10] while
the homoleptic siloxides M(OSiR3)4 [R = OtBu,11,12 Ph13]
are known for all members of the group. The analogous
tetra(boroxide), Hf{OB(mes)2}4 (4), also crystallises as the
monomeric species‡ (Fig. 2), contrasting with the only other
heteroleptic boroxide, [Fe{OB(mes)2}{l-OB(mes)2}]2, which
contains both terminal and bridging boroxides.5 The intra-
ligand angles at hafnium [range 107.9(3)–111.7(4)°] deviate
only slightly from ideal tetrahedral geometry, suggesting a
relatively unstrained system. This is likely due to the boron
atom displacing the mesityl substituents from the metal centre
which effectively reduces the cone angle of the ligand, and
the interlocking, parallel orientation of the aryl moieties from
different ligands (Fig 2b). The Hf–O bond lengths [1.902(7) and
1.916(7) Å] tend towards the low end of the range observed in
the tetrasiloxides [1.935(4)–1.949(4) Å]11 and the tris(alkoxide)
species, Hf{OAr}3Cl [Ar = 2,6-tBuC6H3: 1.917(3)–1.938(3) Å],14
although in the absence of solution-state data, these effects
may derive from purely steric factors rather than be electronic
in origin.
general formula M{OB(mes)2}2R2 [M = group
4 metal,
R = Me, CH2Ph].8 Reaction time was found to be critical
when employing M(CH2Ph)4 as the metal reagent, and
allowing a reaction to stir for extended periods frequently
resulted in the introduction of a greater number of boroxide
ligands than warranted by the stoichiometry of the reaction.
It was established that reacting a 2:1 and 3:1 ratio of
(mes)2BOH and Ti(CH2Ph)4 for 3 h and 15 h, respectively,
proved optimal for the synthesis of Ti{OB(mes)2}2(CH2Ph)2
(1) and Ti{OB(mes)2}3(CH2Ph) (2).† Identical conditions
employing Hf(CH2Ph)4, however, resulted in isolation of the
tris- and tetrakis-boroxides Hf{OB(mes)2}3(CH2Ph) (3) and
Hf{OB(mes)2}4 (4) rather than the anticipated analogues.
The tris(boroxide) mono(benzyl) compound 2 crystallised as
the monomeric species,‡ with three terminal boroxide ligands
and the benzyl group forming a distorted tetrahedral array
about the central titanium atom (Fig 1). The Ti–O distances
[av. 1.794 Å] are notably shorter than in the related d0-titanium
boroxide, Ti(NtBu){OB(mes)2}2(Py)2,3 [av. 1.930 Å], although
the validity of direct comparison is tempered by the presence of
The bis(boroxide) species, Ti{OB(mes)2}2(CH2Ph)2 (1),
constitutes the most promising precursor for the generation
of [ML2R]+ cations, widely believed to be the active species in
olefin polymerisation catalysis. The reaction between 1 and the
well-defined borane activator, B(C6F5)3, generated a single new
species (5), characterised spectroscopically in solution (Fig. 3).
The 1H NMR spectrum showed the appearance of two signals
corresponding to the CH2Ph protons [d 3.54 and 3.39], each of
3 4 2 8
D a l t o n T r a n s . , 2 0 0 4 , 3 4 2 8 – 3 4 3 0
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

View Article Online
Fig. 2 (a) Molecular structure of 4 (ellipsoids drawn at the 30% probability level, hydrogen atoms omitted). Selected bond lengths (Å) and
angles (°): Hf–O1 1.902(7), Hf–O2 1.916(7); Hf–O1–B1 167.7(7), Hf–O2–B2 160.0(7). (b) Spacefill model of 4 (generated from the same projection).
which are shifted down-field from the corresponding singlet
in the neutral dialkyl precursor [d 2.85]. The higher frequency
resonance appears as an unresolved multiplet which collapses
We wish to thank Mr Gregory Knapp for the crystallisation
of compound 2, and Dr A. G. Avent for helpful discussions
with NMR data.
1
to a broadened singlet in the H{11B} experiment, indicating
that this resonance corresponds to a boron bound benzyl group
(2JBH = 9 Hz). We attribute this signal to the formation of
the anion [B(C6F5)3(CH2Ph)]−, commensurate with the desired
alkyl abstraction process. The aromatic resonances associated
with this component appear at higher field than observed for
the free ion (Fig. 3), indicative of an g6-interaction between the
titanium cation and the boroxide anion.15 Such interactions
have previously been observed for group 4 systems employ-
ing aryloxide ligands,6,7 further demonstrating the similarity
between the two ligand systems.
Notes and references
† Selected spectroscopic data (C6D6, 298 K): 1, 1H NMR: d 6.95 (t,
4H, C6H5), 6.82 (t, 4H, C6H5), 6.80 (s, 8H, C6H2), 6.72 (s, 2H, C6H5),
2.85 (s, 4H, CH2), 2.28 (s, 24H, 2,6-Me2), 2.26 (s, 12H, 4-Me). 13C NMR:
d 142.7 (C), 141.1 (CH), 138.8 (CH), 129.0 (CH), 128.8 (C), 128.0 (C),
124.5 (CH), 93.2 (CH2), 23.0 (CH3), 21.4 (CH3). 2, 1H NMR: d 6.87 (d,
2H, C6H5), 6.78 (t, 1H, C6H5), 6.65 (s, 12H, C6H2), 6.52 (d, 2H, C6H5),
3.18 (s, 2H, CH2), 2.25 (s, 36H, 2,6-Me2), 2.12 (s, 24H, 4-Me). 13C NMR:
d 142.7 (C), 141.0 (CH), 138.7 (CH), 129.0 (CH), 128.9 (C), 128.8 (C),
1
124.4 (CH), 93.2 (CH2), 23.0 (CH3), 21.4 (CH3). 4, H NMR: d 6.62
(s, 16H, C6H2), 2.21 (s, 48H, 2,6-Me2) 2.12 (s, 24H, 4-Me). 13C NMR:
140.6 (C), 139.4 (br C), 138.2 (C), 127.3 (CH), 22.6 (CH3), 21.3 (CH3).
1
5, H NMR: d 7.02 (t, 2H, TiCH2Ph), 6.98 (d, 2H, TiCH2Ph), 6.73 (t,
1H, TiCH2Ph), 6.65 (s, 8H, C6H2), 6.45 (d, 2H, BCH2Ph), 6.19 (t, 1H,
BCH2Ph), 5.71 (t, 2H, BCH2Ph), 3.54 (br q, 2H CH2), 3.39 (s, 2H CH2),
2.15 (s, 24H 2,6-Me2), 2.08 (s, 12H, 4-Me). 13C NMR: d 149.6 (C), 145.9
(C), 140.1 (C), 140.6 (d, C6F5), 138.5 (d, C6F5), 136.5 (d, C6F5), 136.2
(o-BCH2Ph), 129.2 (m-C6H2), 129.1 (p-BCH2Ph), 128.9 (m-TiCH2Ph),
128.3 (o-TiCH2Ph), 127.9 (p-TiCH2Ph), 124.0 (m-BCH2Ph), 98.3
(TiCH2Ph), *, 22.8 (C6H2Me3), 21.2 (C6H2Me3) [* resonance for BCH2Ph
was not observed, presumably due to quadrupolar broadening].
‡ Crystallographic data for 2: C61H73B3O3Ti·(C5H12), M = 1006.67,
T = 173(2) K, triclinic, space group P1 (No. 2), a = 14.0992(7),
b = 14.5950(7), c = 17.1898(9) Å, a = 93.847(2), b = 95.609(2), c =
118.391(2)°, U = 3070.8(3) Å3, Z = 2, Dc = 1.09 Mg m−3, l (Mo–Ka) =
0.18 mm−1, independent reflections = 10802 (Rint = 0.053), R1 [for
7204 reflections with I > 2r(I)] = 0.062, wR2 (all data) = 0.163.
Crystallographic data for 4: C72H88B4HfO4, M = 1239.15, T = 173(2) K,
monoclinic, space group C2/c (No. 15), a = 23.147(2), b = 14.8478(7),
c = 21.429(2) Å, b = 119.350(3), U = 6419.4(9) Å3, Z = 4, Dc =
1.28 Mg m−3, l (Mo–Ka) = 1.67 mm−1, independent reflections =
4414 (Rint = 0.193), R1 [for 3036 reflections with I > 2r(I)] = 0.070,
wR2 (all data) = 0.141. CCDC reference numbers 250060 and 250061.
See http://www.rsc.org/suppdata/dt/b4/b414155e/ for crystallographic
data in CIF or other electronic format.
Fig. 3 1H NMR spectrum (C6D6, 300 MHz) of [Ti{OB(mes)2}2-
(CH2Ph)][B(C6F5)3(CH2Ph)] (5) highlighting resonances corresponding
to the g6-bound anion.
The ability of cationic metal systems to polymerise a-olefins
is influenced by the strength of the cation–anion interaction,
where displacement of the latter from the coordination sphere
is generally necessary before propagation is able to proceed.
Given the observed formation of the ion-pair in solution
described above, it was perhaps unsurprising that exposure of
a toluene solution of 5 to ethylene (7.5 bar) did not result in
the formation of polyethylene. Additional complications with
the aforementioned Ti/Zr alkoxide systems is the formation of
a stable mono-insertion product with a range of unsaturated
substrates, affording stable (inactive) cations.7 The possible
formation of insertion products, in addition to the generation
of cationic species from these well-defined precatalysts with
alternative activators forms a part of our ongoing study into
this area, and will be reported in due course.
1 G. J. P. Britovsek, V. C. Gibson and D. F. Wass, Angew. Chem., Int.
Ed., 1999, 38, 428; V. C. Gibson and S. K. Spitzmesser, Chem. Rev.,
2003, 103, 283.
2 T. H. Warren, R. R. Schrock and W. M. Davis, Organometallics,
1996, 15, 562; T. H. Warren, R. R. Schrock and W. M. Davis,
Organometallics, 1998, 17, 308.
3 S. C. Cole, M. P. Coles and P. B. Hitchcock, J. Chem. Soc., Dalton
Trans., 2002, 4168.
4 K. J. Weese, R. A. Bartlett, B. D. Murray, M. M. Olmstead and
P. P. Power, Inorg. Chem., 1987, 26, 2409.
5 H. Chen, P. P. Power and S. C. Shoner, Inorg. Chem., 1991, 30,
2884.
6 M. G. Thorn, Z. C. Etheridge, P. E. Fanwick and I. P. Rothwell,
Organometallics, 1998, 17, 3636.
7 M. G. Thorn, Z. C. Etheridge, P. E. Fanwick and I. P. Rothwell,
J. Organomet. Chem., 1999, 591, 148.
D a l t o n T r a n s . , 2 0 0 4 , 3 4 2 8 – 3 4 3 0
3 4 2 9

View Article Online
8 S. C. Cole, M. P. Coles and P. B. Hitchcock, Organometallics,
2004, DOI:10.1021/om049520j; S. J. Birch, S. R. Boss, S. C. Cole,
M. P. Coles, R. Haigh, P. B. Hitchcock and A. E. H. Wheatley,
Dalton Trans., 2004, 3568–3574.
9 L. D. Durfee, S. L. Latesky, I. P. Rothwell, J. C. Huffman
and K. Folting, Inorg. Chem., 1985, 24, 4569; R. Minhas,
R. Duchateau, S. Gambarotta and C. Bensimon, Inorg. Chem.,
1992, 31, 4933.
12 M. P. Coles, C. G. Lugmair, K. W. Terry and T. D. Tilley, Chem.
Mater., 2000, 12, 122.
13 B. F. G. Johnson, M. C. Klunduk, C. M. Martin, G. Sankar, S. J.
Teate and J. M. Thomas, J. Organomet. Chem., 2000, 596, 221.
14 L. Chamberlain, J. C. Huffman, J. Keddington and I. P. Rothwell,
J. Chem. Soc., Chem. Commun., 1982, 805; S. L. Latesky,
J. Keddington, A. K. McMullen, I. P. Rothwell and J. C. Huffman,
Inorg. Chem., 1985, 24, 995.
10 R. T. Toth and D. W. Stephan, Can. J. Chem., 1991, 69, 172.
11 K. W. Terry, C. G. Lugmair and T. D. Tilley, J. Am. Chem. Soc.,
1997, 119, 9745.
15 C. Pellecchia, A. Grassi and A. Immirzi, J. Am. Chem. Soc., 1993,
115, 1160; M. Bochmann, S. J. Lancaster, M. B. Hursthouse and
K. M. Abdul Malik, Organometallics, 1994, 13, 2235.
3 4 3 0
D a l t o n T r a n s . , 2 0 0 4 , 3 4 2 8 – 3 4 3 0