Cationic organoscandium β-diketiminato chemistry: Arene exchange kinetics in solvent separated ion pairs

Article abstract of DOI:10.1021/ja034680s

Abstraction of methide from the β-diketiminato supported organoscandium complex [L¹ScMe₂]₂ using the trityl borate activator [Ph₃C][B(C₆F₅)₄] in arene solvents gives solvent separated ion pairs in which the arene (C₆H₅Br, 1a; C₆H₆, 1b; C₇H₈, 1c; 1,3,5-Me₃C₆H₃, 1d) is coordinated to the cationic scandium center in an η⁶ bonding mode. L¹ incorporates methyl groups in the 2,4 positions of the ligand backbone and bulky 2,6-diisopropylphenyl groups on the nitrogen atoms. The relative binding strength of the arenes is C₆H₅Br ? C₆H₆ < 1,3,5-Me₃C₆H₃ < C₇H₈. Ion pairs 1a and 1c have been characterized crystallographically, and the C₆H₅Br derivative is notable for its η⁶ bonding mode in preference to the more common η¹ bonding mode via the halogen atom. The kinetics of displacement of mesitylene by toluene (1d → 1c) yield activation parameters of ΔH _≠ = 21.4(6) kcal mol^-1 and ΔS_≠ 6(1) cal mol^-1 K^-1. In combination with the observed lack of dependence of [toluene] on the rate of displacement, these data suggest a mechanism involving partial dissociation of the coordinated arene, followed by attack of the incoming arene. This chemistry has relevance to the role of these solvent separated ion pairs in olefin polymerization processes and presents a rare opportunity for the detailed study of these ephemeral species. Copyright

Full text of DOI:10.1021/ja034680s

Published on Web 04/18/2003
Cationic Organoscandium â-Diketiminato Chemistry: Arene Exchange
Kinetics in Solvent Separated Ion Pairs
Paul G. Hayes, Warren E. Piers,* and Masood Parvez
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta, T2N 1N4, Canada
Received February 14, 2003; E-mail: wpiers@ucalgary.ca
Scheme 1
It is well established that early transition metal organometallic
ion pairs with weakly coordinating anions are the active species in
olefin polymerization processes.1 Increasingly, it is becoming
apparent that subtle interplay between cation/anion interactions and
solvent effects has a significant impact on ion pair dynamics, which
in turn profoundly influence the activity and selectivity of a given
catalyst system.2 While these effects have been studied experimen-
tally and computationally in some detail for metallocene3 and
constrained geometry catalysts families,4 some controversy remains
concerning the relative importance of contact ion pairs (CIP) versus
solvent separated ion pairs (SSIP) in the initiation and propagation
steps of a polymerization reaction.5 Of the two classes of ion pair,
the CIPs are the more well understood from a structural and
dynamic perspective. SSIPs formed in relevant nonpolar solvents
are more ephemeral species, with few experimentally well-
characterized examples in the literature.6
Recently, we reported a family of base-free dialkyl organoscan-
dium complexes supported by bulky â-diketiminato ligands incor-
porating 2,6-diisopropylphenyl groups on nitrogen, and either Me
t
(L1) or Bu (L2) substituents in the 2,4 positions of the ligand
t
backbone.7 Activation of the monomeric Bu-substituted dimethyl
compound L2ScMe2 with B(C6F5)3 gave a well-defined CIP which
was highly active for ethylene polymerization.8 Here we describe
the activation chemistry of the less sterically encumbered scandium
Figure 1. Molecular structure of 1a (counterion omitted for clarity).
Selected bond distances (Å) and angles (deg): Sc(1)-N(1), 2.100(4); Sc(1)-
N(2), 2.105(4); Sc(1)-C(1), 2.162(5); Sc(1)-(C32), 2.842(4); Sc(1)-C(33),
2.767(4); Sc(1)-C(34), 2.682(4); Sc(1)-C(35), 2.640(4); Sc(1)-C(36),
2.715(4); Sc(1)-C(37), 2.802(5); N(1)-Sc(1)-N(2), 90.79(14); N(1)-
Sc(1)-C(1), 105.22(16); N(2)-Sc(1)-C(1), 105.23(16).
dimethyl derivative supported by L1, which gives rise to stable
SSIPs in arene solvents, providing the opportunity to probe solvent
exchange processes in these rare species.
Previously, the compounds L1ScCl2 and L1ScMe2 were only
available as THF adducts,7 but we have subsequently discovered
that prolonged exposure of L1ScCl2‚THF to 10-4 Torr at 130 °C
removes the base completely. Alkylation of the resulting oligomeric
[L1ScCl2]n with MeLi in toluene cleanly affords the base-free
dimethyl derivative [L1ScMe2]2 in 90% yield. The dimethyl
compound is dimeric in the solid state, but, in solution, the terminal
and bridging methyl groups are not distinguishable by NMR
spectroscopy, suggesting a rapid dimer/monomer equilibrium or
an intramolecular exchange process. In any event, reaction of
[L1ScMe2]2 with common activators yields products consistent with
reaction through a monomeric organoscandium compound.
For example, reaction with 1 equiv of B(C6F5)39 at 240 K showed
the formation of the expected CIP [L1ScMe]+[MeB(C6F5)3]-. Upon
warming to 270 K, however, rapid C6F5 transfer from the borate
counterion to the metal center, along with production of MeB-
(C6F5)2, was observed.10 Addition of a second equivalent of borane
results in abstraction of the remaining methyl group and formation
of the CIP [L1Sc-C6F5][MeB(C6F5)3]. Evidently, the lower steric
impact of L1 versus L2 reduces the barrier for C6F5 back-transfer
because we do not observe this in the [L2ScMe][MeB(C6F5)3] CIP.8
By contrast, activation of [L1ScMe2]2 in d5-bromobenzene with
1 equiv of the trityl borate activator, [CPh3][B(C6F5)4],11 cleanly
produces an ion pair (1a) which shows no evidence for C6F5 transfer
even at elevated temperatures (Scheme 1). X-ray crystallography
reveals that in 1a the bromobenzene solvent, surprisingly, coordi-
nates the cationic Sc center in an η6 bonding mode (Figure 1).
Usually, haloarenes coordinate in an η1 fashion via the halogen to
d0 metals;12 the preference for η6 hapticity here demonstrates the
high electrophilicity and steric openness of these cations. Although
the aromatic fragment is η6 bound, steric interactions invoke
significant ring tilting (Sc-Carene ) 2.640(4)-2.842(4) Å).13 There
are no close contacts (e4.9 Å) between the borate anion and the
metal center.
The 1H NMR spectra for SSIP 1a at various temperatures exhibit
a pattern consistent with rapid exchange between free and bound
arene. Indeed, upon addition of more basic arenes to 1a, the
bromobenzene is displaced to give new SSIPs 1b-d (Scheme 1).14
X-ray crystallography shows that the toluene adduct 1c is essentially
isostructural with 1a,15 but the increased basicity of toluene16 leads
to Cs symmetry in the 1H NMR spectrum at ambient temperatures.
Upfield shifted resonances for the η6 toluene are observed at 6.81,
6.66, 6.39, and 1.85 ppm in C6D5Br. Addition of excess d8-toluene
to this sample of 1c results in the disappearance of the signals for
coordinated toluene over the course of 5 min, indicating exchange
with free toluene. Competition experiments show that the order of
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C O M M U N I C A T I O N S
concrete explanation for this observation. Indeed, preliminary
studies using mesitylene complex 1d as a catalyst show that there
is significant polymerization activity when the experiment is
conducted in bromobenzene whereas activity is negligible when
carried out in more coordinating toluene. Thus, while ethylene is
able to displace mesitylene, which has a barrier similar to that of
the coordinating anion [MeB(C6F5)3]-, it cannot compete with
toluene for the active site, and activity is nullified.
Acknowledgment. This work is dedicated to Prof. Hans H.
Brintzinger for his outstanding contributions to organometallic
chemistry. Financial support for this work came from the NSERC
of Canada in the form of a Discovery Grant and an E. W. R. Steacie
Fellowship to W.E.P. (2001-2003), and scholarship support to
P.G.H. (PGS-A and PGS-B). P.G.H. also thanks the Alberta
Heritage Foundation for a Steinhauer Award and the Sir Izaak
Walton Killam Foundation for a Fellowship.
Supporting Information Available: Experimental details, tables
of crystal data, atomic coordinates, bond lengths and angles, and
anisotropic parameters for 1a and 1c (PDF and CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
References
Figure 2. Representative series of 1H NMR spectra (300 MHz, 270 K,
C6D5Br) for the arene exchange of 1d to 1c (left). Proposed mechanism of
exchange (right).
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